diff --git a/includes/amp-helper-functions.php b/includes/amp-helper-functions.php
index 67cc7878c09..e75beaca5b9 100644
--- a/includes/amp-helper-functions.php
+++ b/includes/amp-helper-functions.php
@@ -915,6 +915,7 @@ function amp_get_content_sanitizers( $post = null ) {
 		'AMP_Style_Sanitizer'             => [
 			'include_manifest_comment' => ( defined( 'WP_DEBUG' ) && WP_DEBUG ) ? 'always' : 'when_excessive',
 		],
+		'AMP_Meta_Sanitizer'              => [],
 		'AMP_Tag_And_Attribute_Sanitizer' => [], // Note: This whitelist sanitizer must come at the end to clean up any remaining issues the other sanitizers didn't catch.
 	];
 
@@ -971,7 +972,7 @@ function amp_get_content_sanitizers( $post = null ) {
 	}
 
 	// Force style sanitizer and whitelist sanitizer to be at end.
-	foreach ( [ 'AMP_Style_Sanitizer', 'AMP_Tag_And_Attribute_Sanitizer' ] as $class_name ) {
+	foreach ( [ 'AMP_Style_Sanitizer', 'AMP_Meta_Sanitizer', 'AMP_Tag_And_Attribute_Sanitizer' ] as $class_name ) {
 		if ( isset( $sanitizers[ $class_name ] ) ) {
 			$sanitizer = $sanitizers[ $class_name ];
 			unset( $sanitizers[ $class_name ] );
diff --git a/includes/class-amp-autoloader.php b/includes/class-amp-autoloader.php
index 80b0fe4e765..c6241e87671 100644
--- a/includes/class-amp-autoloader.php
+++ b/includes/class-amp-autoloader.php
@@ -78,6 +78,7 @@ class AMP_Autoloader {
 		'AMP_Iframe_Sanitizer'               => 'includes/sanitizers/class-amp-iframe-sanitizer',
 		'AMP_Img_Sanitizer'                  => 'includes/sanitizers/class-amp-img-sanitizer',
 		'AMP_Link_Sanitizer'                 => 'includes/sanitizers/class-amp-link-sanitizer',
+		'AMP_Meta_Sanitizer'                 => 'includes/sanitizers/class-amp-meta-sanitizer',
 		'AMP_Nav_Menu_Toggle_Sanitizer'      => 'includes/sanitizers/class-amp-nav-menu-toggle-sanitizer',
 		'AMP_Nav_Menu_Dropdown_Sanitizer'    => 'includes/sanitizers/class-amp-nav-menu-dropdown-sanitizer',
 		'AMP_Comments_Sanitizer'             => 'includes/sanitizers/class-amp-comments-sanitizer',
diff --git a/includes/class-amp-theme-support.php b/includes/class-amp-theme-support.php
index 0df3b892bd4..b9fe763e303 100644
--- a/includes/class-amp-theme-support.php
+++ b/includes/class-amp-theme-support.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_Theme_Support
  *
@@ -1600,10 +1602,10 @@ public static function filter_admin_bar_script_loader_tag( $tag, $handle ) {
 	 * @link https://amp.dev/documentation/guides-and-tutorials/optimize-and-measure/optimize_amp/
 	 * @todo All of this might be better placed inside of a sanitizer.
 	 *
-	 * @param DOMDocument $dom            Document.
-	 * @param string[]    $script_handles AMP script handles for components identified during output buffering.
+	 * @param Document $dom            Document.
+	 * @param string[] $script_handles AMP script handles for components identified during output buffering.
 	 */
-	public static function ensure_required_markup( DOMDocument $dom, $script_handles = [] ) {
+	public static function ensure_required_markup( Document $dom, $script_handles = [] ) {
 		/**
 		 * Elements.
 		 *
@@ -1614,23 +1616,14 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 		 * @var DOMElement $noscript
 		 */
 
-		$xpath = new DOMXPath( $dom );
-
-		// Make sure the HEAD element is in the doc.
-		$head = $dom->getElementsByTagName( 'head' )->item( 0 );
-		if ( ! $head ) {
-			$head = $dom->createElement( 'head' );
-			$dom->documentElement->insertBefore( $head, $dom->documentElement->firstChild );
-		}
-
 		// Ensure there is a schema.org script in the document.
 		// @todo Consider applying the amp_schemaorg_metadata filter on the contents when a script is already present.
-		$schema_org_meta_script = $xpath->query( '//script[ @type = "application/ld+json" ][ contains( ./text(), "schema.org" ) ]' )->item( 0 );
+		$schema_org_meta_script = $dom->xpath->query( '//script[ @type = "application/ld+json" ][ contains( ./text(), "schema.org" ) ]' )->item( 0 );
 		if ( ! $schema_org_meta_script ) {
 			$script = $dom->createElement( 'script' );
 			$script->setAttribute( 'type', 'application/ld+json' );
 			$script->appendChild( $dom->createTextNode( wp_json_encode( amp_get_schemaorg_metadata(), JSON_UNESCAPED_UNICODE ) ) );
-			$head->appendChild( $script );
+			$dom->head->appendChild( $script );
 		}
 
 		// Gather all links.
@@ -1647,7 +1640,7 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 				),
 			],
 		];
-		$link_elements = $head->getElementsByTagName( 'link' );
+		$link_elements = $dom->head->getElementsByTagName( 'link' );
 		foreach ( $link_elements as $link ) {
 			if ( $link->hasAttribute( 'rel' ) ) {
 				$links[ $link->getAttribute( 'rel' ) ][] = $link;
@@ -1665,97 +1658,18 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 					'href' => self::get_current_canonical_url(),
 				]
 			);
-			$head->appendChild( $rel_canonical );
-		}
-
-		/*
-		 * Ensure meta charset and meta viewport are present.
-		 *
-		 * "AMP is already quite restrictive about which markup is allowed in the <head> section. However,
-		 * there are a few basic optimizations that you can apply. The key is to structure the <head> section
-		 * in a way so that all render-blocking scripts and custom fonts load as fast as possible."
-		 *
-		 * "1. The first tag should be the meta charset tag, followed by any remaining meta tags."
-		 *
-		 * {@link https://amp.dev/documentation/guides-and-tutorials/optimize-and-measure/optimize_amp/ Optimize the AMP Runtime loading}
-		 */
-		$meta_charset         = null;
-		$meta_viewport        = null;
-		$meta_amp_script_srcs = [];
-		$meta_elements        = [];
-		foreach ( $head->getElementsByTagName( 'meta' ) as $meta ) {
-			if ( $meta->hasAttribute( 'charset' ) ) { // There will not be a meta[http-equiv] because the sanitizer removed it.
-				$meta_charset = $meta;
-			} elseif ( 'viewport' === $meta->getAttribute( 'name' ) ) {
-				$meta_viewport = $meta;
-			} elseif ( 'amp-script-src' === $meta->getAttribute( 'name' ) ) {
-				$meta_amp_script_srcs[] = $meta;
-			} else {
-				$meta_elements[] = $meta;
-			}
-		}
-
-		// Handle meta charset.
-		if ( ! $meta_charset ) {
-			// Warning: This probably means the character encoding needs to be converted.
-			$meta_charset = AMP_DOM_Utils::create_node(
-				$dom,
-				'meta',
-				[
-					'charset' => 'utf-8',
-				]
-			);
-		} else {
-			$head->removeChild( $meta_charset ); // So we can move it.
-		}
-		$head->insertBefore( $meta_charset, $head->firstChild );
-
-		// Handle meta viewport.
-		if ( ! $meta_viewport ) {
-			$meta_viewport = AMP_DOM_Utils::create_node(
-				$dom,
-				'meta',
-				[
-					'name'    => 'viewport',
-					'content' => 'width=device-width',
-				]
-			);
-		} else {
-			$head->removeChild( $meta_viewport ); // So we can move it.
+			$dom->head->appendChild( $rel_canonical );
 		}
-		$head->insertBefore( $meta_viewport, $meta_charset->nextSibling );
 
-		// Handle meta amp-script-src elements.
-		$first_meta_amp_script_src = array_shift( $meta_amp_script_srcs );
-		if ( $first_meta_amp_script_src ) {
-			$meta_elements[] = $first_meta_amp_script_src;
-
-			// Merge (and remove) any subsequent meta amp-script-src elements.
-			if ( ! empty( $meta_amp_script_srcs ) ) {
-				$content_values = [ $first_meta_amp_script_src->getAttribute( 'content' ) ];
-				foreach ( $meta_amp_script_srcs as $meta_amp_script_src ) {
-					$meta_amp_script_src->parentNode->removeChild( $meta_amp_script_src );
-					$content_values[] = $meta_amp_script_src->getAttribute( 'content' );
-				}
-				$first_meta_amp_script_src->setAttribute( 'content', implode( ' ', $content_values ) );
-				unset( $meta_amp_script_src, $content_values );
-			}
-		}
-		unset( $meta_amp_script_srcs, $first_meta_amp_script_src );
-
-		// Insert all the the meta elements next in the head.
-		$previous_node = $meta_viewport;
-		foreach ( $meta_elements as $meta_element ) {
-			$meta_element->parentNode->removeChild( $meta_element );
-			$head->insertBefore( $meta_element, $previous_node->nextSibling );
-			$previous_node = $meta_element;
-		}
+		// Store the last meta tag as the previous node to append to.
+		$meta_tags     = $dom->head->getElementsByTagName( 'meta' );
+		$previous_node = $meta_tags->length > 0 ? $meta_tags->item( $meta_tags->length - 1 ) : $dom->head->firstChild;
 
 		// Handle the title.
-		$title = $head->getElementsByTagName( 'title' )->item( 0 );
+		$title = $dom->head->getElementsByTagName( 'title' )->item( 0 );
 		if ( $title ) {
 			$title->parentNode->removeChild( $title ); // So we can move it.
-			$head->insertBefore( $title, $previous_node->nextSibling );
+			$dom->head->insertBefore( $title, $previous_node->nextSibling );
 			$previous_node = $title;
 		}
 
@@ -1771,7 +1685,7 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 		$ordered_scripts = [];
 		$head_scripts    = [];
 		$runtime_src     = wp_scripts()->registered['amp-runtime']->src;
-		foreach ( $head->getElementsByTagName( 'script' ) as $script ) { // Note that prepare_response() already moved body scripts to head.
+		foreach ( $dom->head->getElementsByTagName( 'script' ) as $script ) { // Note that prepare_response() already moved body scripts to head.
 			$head_scripts[] = $script;
 		}
 		foreach ( $head_scripts as $script ) {
@@ -1879,7 +1793,7 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 				if ( $link->parentNode ) {
 					$link->parentNode->removeChild( $link ); // So we can move it.
 				}
-				$head->insertBefore( $link, $previous_node->nextSibling );
+				$dom->head->insertBefore( $link, $previous_node->nextSibling );
 				$previous_node = $link;
 			}
 		}
@@ -1913,25 +1827,25 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 		 */
 		$ordered_scripts = array_merge( $ordered_scripts, $amp_scripts );
 		foreach ( $ordered_scripts as $ordered_script ) {
-			$head->insertBefore( $ordered_script, $previous_node->nextSibling );
+			$dom->head->insertBefore( $ordered_script, $previous_node->nextSibling );
 			$previous_node = $ordered_script;
 		}
 
 		/*
 		 * "8. Specify any custom styles by using the <style amp-custom> tag."
 		 */
-		$style = $xpath->query( './style[ @amp-custom ]', $head )->item( 0 );
+		$style = $dom->xpath->query( './style[ @amp-custom ]', $dom->head )->item( 0 );
 		if ( $style ) {
 			// Ensure the CSS manifest comment remains before style[amp-custom].
 			if ( $style->previousSibling instanceof DOMComment ) {
 				$comment = $style->previousSibling;
 				$comment->parentNode->removeChild( $comment );
-				$head->insertBefore( $comment, $previous_node->nextSibling );
+				$dom->head->insertBefore( $comment, $previous_node->nextSibling );
 				$previous_node = $comment;
 			}
 
 			$style->parentNode->removeChild( $style );
-			$head->insertBefore( $style, $previous_node->nextSibling );
+			$dom->head->insertBefore( $style, $previous_node->nextSibling );
 			$previous_node = $style;
 		}
 
@@ -1944,7 +1858,7 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 		 * "10. Finally, specify the AMP boilerplate code. By putting the boilerplate code last, it prevents custom styles
 		 * from accidentally overriding the boilerplate css rules."
 		 */
-		$style = $xpath->query( './style[ @amp-boilerplate ]', $head )->item( 0 );
+		$style = $dom->xpath->query( './style[ @amp-boilerplate ]', $dom->head )->item( 0 );
 		if ( ! $style ) {
 			$style = $dom->createElement( 'style' );
 			$style->setAttribute( 'amp-boilerplate', '' );
@@ -1952,9 +1866,9 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 		} else {
 			$style->parentNode->removeChild( $style ); // So we can move it.
 		}
-		$head->appendChild( $style );
+		$dom->head->appendChild( $style );
 
-		$noscript = $xpath->query( './noscript[ style[ @amp-boilerplate ] ]', $head )->item( 0 );
+		$noscript = $dom->xpath->query( './noscript[ style[ @amp-boilerplate ] ]', $dom->head )->item( 0 );
 		if ( ! $noscript ) {
 			$noscript = $dom->createElement( 'noscript' );
 			$style    = $dom->createElement( 'style' );
@@ -1964,7 +1878,7 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 		} else {
 			$noscript->parentNode->removeChild( $noscript ); // So we can move it.
 		}
-		$head->appendChild( $noscript );
+		$dom->head->appendChild( $noscript );
 
 		unset( $previous_node );
 	}
@@ -2314,33 +2228,26 @@ public static function prepare_response( $response, $args = [] ) {
 			);
 		}
 
-		$dom   = AMP_DOM_Utils::get_dom( $response );
-		$xpath = new DOMXPath( $dom );
-		$head  = $dom->getElementsByTagName( 'head' )->item( 0 );
+		$dom = Document::from_html( $response );
 
 		// Move anything after </html>, such as Query Monitor output added at shutdown, to be moved before </body>.
-		$body = $dom->getElementsByTagName( 'body' )->item( 0 );
-		if ( $body ) {
-			while ( $dom->documentElement->nextSibling ) {
-				// Trailing elements after </html> will get wrapped in additional <html> elements.
-				if ( 'html' === $dom->documentElement->nextSibling->nodeName ) {
-					while ( $dom->documentElement->nextSibling->firstChild ) {
-						$body->appendChild( $dom->documentElement->nextSibling->firstChild );
-					}
-					$dom->removeChild( $dom->documentElement->nextSibling );
-				} else {
-					$body->appendChild( $dom->documentElement->nextSibling );
+		while ( $dom->documentElement->nextSibling ) {
+			// Trailing elements after </html> will get wrapped in additional <html> elements.
+			if ( 'html' === $dom->documentElement->nextSibling->nodeName ) {
+				while ( $dom->documentElement->nextSibling->firstChild ) {
+					$dom->body->appendChild( $dom->documentElement->nextSibling->firstChild );
 				}
+				$dom->removeChild( $dom->documentElement->nextSibling );
+			} else {
+				$dom->body->appendChild( $dom->documentElement->nextSibling );
 			}
 		}
 
 		AMP_HTTP::send_server_timing( 'amp_dom_parse', -$dom_parse_start, 'AMP DOM Parse' );
 
 		// Make sure scripts from the body get moved to the head.
-		if ( isset( $head ) ) {
-			foreach ( $xpath->query( '//body//script[ @custom-element or @custom-template or @src = "https://cdn.ampproject.org/v0.js" ]' ) as $script ) {
-				$head->appendChild( $script->parentNode->removeChild( $script ) );
-			}
+		foreach ( $dom->xpath->query( '//body//script[ @custom-element or @custom-template or @src = "https://cdn.ampproject.org/v0.js" ]' ) as $script ) {
+			$dom->head->appendChild( $script->parentNode->removeChild( $script ) );
 		}
 
 		// Ensure the mandatory amp attribute is present on the html element.
@@ -2403,11 +2310,9 @@ public static function prepare_response( $response, $args = [] ) {
 				 * Make sure that document.write() is disabled to prevent dynamically-added content (such as added
 				 * via amp-live-list) from wiping out the page by introducing any scripts that call this function.
 				 */
-				if ( $head ) {
-					$script = $dom->createElement( 'script' );
-					$script->appendChild( $dom->createTextNode( 'document.addEventListener( "DOMContentLoaded", function() { document.write = function( text ) { throw new Error( "[AMP-WP] Prevented document.write() call with: "  + text ); }; } );' ) );
-					$head->appendChild( $script );
-				}
+				$script = $dom->createElement( 'script' );
+				$script->appendChild( $dom->createTextNode( 'document.addEventListener( "DOMContentLoaded", function() { document.write = function( text ) { throw new Error( "[AMP-WP] Prevented document.write() call with: "  + text ); }; } );' ) );
+				$dom->head->appendChild( $script );
 			} elseif ( ! self::is_customize_preview_iframe() ) {
 				$response = esc_html__( 'Redirecting to non-AMP version.', 'amp' );
 
@@ -2438,7 +2343,7 @@ public static function prepare_response( $response, $args = [] ) {
 		AMP_Validation_Manager::finalize_validation( $dom );
 
 		$response  = "<!DOCTYPE html>\n";
-		$response .= AMP_DOM_Utils::get_content_from_dom_node( $dom, $dom->documentElement );
+		$response .= $dom->saveHTML( $dom->documentElement );
 
 		AMP_HTTP::send_server_timing( 'amp_dom_serialize', -$dom_serialize_start, 'AMP DOM Serialize' );
 
diff --git a/includes/embeds/class-amp-facebook-embed.php b/includes/embeds/class-amp-facebook-embed.php
index 35ec93960f6..2455496c292 100644
--- a/includes/embeds/class-amp-facebook-embed.php
+++ b/includes/embeds/class-amp-facebook-embed.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_Facebook_Embed_Handler
  */
@@ -99,14 +101,9 @@ public function render( $args ) {
 	/**
 	 * Sanitized <div class="fb-video" data-href=> tags to <amp-facebook>.
 	 *
-	 * @param DOMDocument $dom DOM.
+	 * @param Document $dom DOM.
 	 */
-	public function sanitize_raw_embeds( $dom ) {
-		/**
-		 * Node list.
-		 *
-		 * @var DOMNodeList $node
-		 */
+	public function sanitize_raw_embeds( Document $dom ) {
 		$nodes     = $dom->getElementsByTagName( $this->sanitize_tag );
 		$num_nodes = $nodes->length;
 
@@ -134,9 +131,8 @@ public function sanitize_raw_embeds( $dom ) {
 		 */
 		$fb_root = $dom->getElementById( 'fb-root' );
 		if ( $fb_root ) {
-			$xpath   = new DOMXPath( $dom );
 			$scripts = [];
-			foreach ( $xpath->query( '//script[ starts-with( @src, "https://connect.facebook.net" ) and contains( @src, "sdk.js" ) ]' ) as $script ) {
+			foreach ( $dom->xpath->query( '//script[ starts-with( @src, "https://connect.facebook.net" ) and contains( @src, "sdk.js" ) ]' ) as $script ) {
 				$scripts[] = $script;
 			}
 			foreach ( $scripts as $script ) {
@@ -152,7 +148,7 @@ public function sanitize_raw_embeds( $dom ) {
 	 * @param DOMElement $node The DOMNode to adjust and replace.
 	 * @return string|null Embed type or null if not detected.
 	 */
-	private function get_embed_type( $node ) {
+	private function get_embed_type( DOMElement $node ) {
 		$class_attr = $node->getAttribute( 'class' );
 		if ( null === $class_attr || ! $node->hasAttribute( 'data-href' ) ) {
 			return null;
@@ -188,11 +184,11 @@ private function get_embed_type( $node ) {
 	/**
 	 * Create amp-facebook and replace node.
 	 *
-	 * @param DOMDocument $dom        The HTML Document.
-	 * @param DOMElement  $node       The DOMNode to adjust and replace.
-	 * @param string      $embed_type Embed type.
+	 * @param Document   $dom        The HTML Document.
+	 * @param DOMElement $node       The DOMNode to adjust and replace.
+	 * @param string     $embed_type Embed type.
 	 */
-	private function create_amp_facebook_and_replace_node( $dom, $node, $embed_type ) {
+	private function create_amp_facebook_and_replace_node( Document $dom, DOMElement $node, $embed_type ) {
 
 		$attributes = [
 			'layout' => 'responsive',
diff --git a/includes/embeds/class-amp-gallery-embed.php b/includes/embeds/class-amp-gallery-embed.php
index 5ef93120495..a0d4fc0c5a1 100644
--- a/includes/embeds/class-amp-gallery-embed.php
+++ b/includes/embeds/class-amp-gallery-embed.php
@@ -5,7 +5,8 @@
  * @package AMP
  */
 
-use Amp\AmpWP\Component\DOMElementList;
+use Amp\AmpWP\Dom\Document;
+use Amp\AmpWP\Dom\ElementList;
 use Amp\AmpWP\Component\Carousel;
 
 /**
@@ -203,7 +204,7 @@ public function maybe_override_gallery( $html, $attributes ) {
 	 * @return string Rendered.
 	 */
 	public function render( $args ) {
-		$dom                        = new DOMDocument();
+		$dom                        = new Document();
 		$this->did_convert_elements = true;
 
 		$args = wp_parse_args(
@@ -217,7 +218,7 @@ public function render( $args ) {
 			return '';
 		}
 
-		$images = new DOMElementList();
+		$images = new ElementList();
 		foreach ( $args['images'] as $props ) {
 			$image_atts = [
 				'src'    => $props['url'],
@@ -261,7 +262,7 @@ public function render( $args ) {
 		// Prevent an error in get_content_from_dom_node() when it calls $node->parentNode->insertBefore().
 		$dom->appendChild( $carousel_node );
 
-		return AMP_DOM_Utils::get_content_from_dom_node( $dom, $carousel_node );
+		return $dom->saveHTML( $carousel_node );
 	}
 
 	/**
diff --git a/includes/embeds/class-amp-instagram-embed.php b/includes/embeds/class-amp-instagram-embed.php
index b41008936fa..0c738bb9838 100644
--- a/includes/embeds/class-amp-instagram-embed.php
+++ b/includes/embeds/class-amp-instagram-embed.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_Instagram_Embed_Handler
  *
@@ -132,9 +134,9 @@ private function get_instagram_id_from_url( $url ) {
 	/**
 	 * Sanitized <blockquote class="instagram-media"> tags to <amp-instagram>
 	 *
-	 * @param DOMDocument $dom DOM.
+	 * @param Document $dom DOM.
 	 */
-	public function sanitize_raw_embeds( $dom ) {
+	public function sanitize_raw_embeds( Document $dom ) {
 		/**
 		 * Node list.
 		 *
@@ -162,8 +164,8 @@ public function sanitize_raw_embeds( $dom ) {
 	/**
 	 * Make final modifications to DOMNode
 	 *
-	 * @param DOMDocument $dom The HTML Document.
-	 * @param DOMElement  $node The DOMNode to adjust and replace.
+	 * @param Document   $dom The HTML Document.
+	 * @param DOMElement $node The DOMNode to adjust and replace.
 	 */
 	private function create_amp_instagram_and_replace_node( $dom, $node ) {
 		$instagram_id = $this->get_instagram_id_from_url( $node->getAttribute( 'data-instgrm-permalink' ) );
diff --git a/includes/embeds/class-amp-twitter-embed-handler.php b/includes/embeds/class-amp-twitter-embed-handler.php
index 4dc87553933..0ea94f35420 100644
--- a/includes/embeds/class-amp-twitter-embed-handler.php
+++ b/includes/embeds/class-amp-twitter-embed-handler.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_Twitter_Embed_Handler
  *
@@ -105,14 +107,9 @@ public function oembed_timeline( $matches ) {
 	/**
 	 * Sanitized <blockquote class="twitter-tweet"> tags to <amp-twitter>.
 	 *
-	 * @param DOMDocument $dom DOM.
+	 * @param Document $dom DOM.
 	 */
-	public function sanitize_raw_embeds( $dom ) {
-		/**
-		 * Node list.
-		 *
-		 * @var DOMNodeList $node
-		 */
+	public function sanitize_raw_embeds( Document $dom ) {
 		$nodes     = $dom->getElementsByTagName( $this->sanitize_tag );
 		$num_nodes = $nodes->length;
 
@@ -152,10 +149,10 @@ private function is_tweet_raw_embed( $node ) {
 	/**
 	 * Make final modifications to DOMNode
 	 *
-	 * @param DOMDocument $dom The HTML Document.
-	 * @param DOMElement  $node The DOMNode to adjust and replace.
+	 * @param Document   $dom The HTML Document.
+	 * @param DOMElement $node The DOMNode to adjust and replace.
 	 */
-	private function create_amp_twitter_and_replace_node( $dom, $node ) {
+	private function create_amp_twitter_and_replace_node( Document $dom, DOMElement $node ) {
 		$tweet_id = $this->get_tweet_id( $node );
 		if ( ! $tweet_id ) {
 			return;
diff --git a/includes/sanitizers/class-amp-audio-sanitizer.php b/includes/sanitizers/class-amp-audio-sanitizer.php
index 6a15c17e86e..21d20aa29c7 100644
--- a/includes/sanitizers/class-amp-audio-sanitizer.php
+++ b/includes/sanitizers/class-amp-audio-sanitizer.php
@@ -42,7 +42,7 @@ public function get_selector_conversion_mapping() {
 	}
 
 	/**
-	 * Sanitize the <audio> elements from the HTML contained in this instance's DOMDocument.
+	 * Sanitize the <audio> elements from the HTML contained in this instance's Dom\Document.
 	 *
 	 * @since 0.2
 	 */
diff --git a/includes/sanitizers/class-amp-base-sanitizer.php b/includes/sanitizers/class-amp-base-sanitizer.php
index b93da28fd99..65c20f5dc6a 100644
--- a/includes/sanitizers/class-amp-base-sanitizer.php
+++ b/includes/sanitizers/class-amp-base-sanitizer.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_Base_Sanitizer
  */
@@ -31,7 +33,7 @@ abstract class AMP_Base_Sanitizer {
 	/**
 	 * DOM.
 	 *
-	 * @var DOMDocument A standard PHP representation of an HTML document in object form.
+	 * @var Document An Amp\Document representation of an HTML document.
 	 *
 	 * @since 0.2
 	 */
@@ -62,7 +64,7 @@ abstract class AMP_Base_Sanitizer {
 
 	/**
 	 * Flag to be set in child class' sanitize() method indicating if the
-	 * HTML contained in the DOMDocument has been sanitized yet or not.
+	 * HTML contained in the Dom\Document has been sanitized yet or not.
 	 *
 	 * @since 0.2
 	 *
@@ -89,8 +91,8 @@ abstract class AMP_Base_Sanitizer {
 	 *
 	 * @since 0.2
 	 *
-	 * @param DOMDocument $dom Represents the HTML document to sanitize.
-	 * @param array       $args {
+	 * @param Document $dom Represents the HTML document to sanitize.
+	 * @param array    $args {
 	 *      Args.
 	 *
 	 *      @type int $content_max_width
@@ -108,7 +110,7 @@ public function __construct( $dom, $args = [] ) {
 		if ( ! empty( $this->args['use_document_element'] ) ) {
 			$this->root_element = $this->dom->documentElement;
 		} else {
-			$this->root_element = $this->dom->getElementsByTagName( 'body' )->item( 0 );
+			$this->root_element = $this->dom->body;
 		}
 	}
 
@@ -199,13 +201,14 @@ public function get_stylesheets() {
 	}
 
 	/**
-	 * Get HTML body as DOMElement from DOMDocument received by the constructor.
+	 * Get HTML body as DOMElement from Dom\Document received by the constructor.
 	 *
-	 * @deprecated Just reference $root_element instead.
+	 * @deprecated Use $this->dom->body instead.
 	 * @return DOMElement The body element.
 	 */
 	protected function get_body_node() {
-		return $this->dom->getElementsByTagName( 'body' )->item( 0 );
+		_deprecated_function( 'Use $this->dom->body instead', '1.5.0' );
+		return $this->dom->body;
 	}
 
 	/**
diff --git a/includes/sanitizers/class-amp-block-sanitizer.php b/includes/sanitizers/class-amp-block-sanitizer.php
index c032b66a135..a9f4e6e5d15 100644
--- a/includes/sanitizers/class-amp-block-sanitizer.php
+++ b/includes/sanitizers/class-amp-block-sanitizer.php
@@ -71,8 +71,7 @@ function ( $matches ) use ( &$responsive_width, &$responsive_height ) {
 			// @todo Should we consider just eliminating the .wp-block-embed__wrapper element since unnecessary?
 			// For visual parity with blocks in non-AMP pages, override the oEmbed's natural responsive dimensions with the aspect ratio specified in the wp-embed-aspect-* class name.
 			if ( $responsive_width && $responsive_height ) {
-				$xpath       = new DOMXPath( $this->dom );
-				$amp_element = $xpath->query( './div[ contains( @class, "wp-block-embed__wrapper" ) ]/*[ @layout = "responsive" ]', $node )->item( 0 );
+				$amp_element = $this->dom->xpath->query( './div[ contains( @class, "wp-block-embed__wrapper" ) ]/*[ @layout = "responsive" ]', $node )->item( 0 );
 				if ( $amp_element instanceof DOMElement ) {
 					$amp_element->setAttribute( 'width', $responsive_width );
 					$amp_element->setAttribute( 'height', $responsive_height );
diff --git a/includes/sanitizers/class-amp-comments-sanitizer.php b/includes/sanitizers/class-amp-comments-sanitizer.php
index b79f2474296..d40353a63f6 100644
--- a/includes/sanitizers/class-amp-comments-sanitizer.php
+++ b/includes/sanitizers/class-amp-comments-sanitizer.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_Comments_Sanitizer
  *
@@ -46,8 +48,7 @@ public function sanitize() {
 		}
 
 		if ( ! empty( $this->args['comments_live_list'] ) ) {
-			$xpath    = new DOMXPath( $this->dom );
-			$comments = $xpath->query( '//amp-live-list/*[ @items ]/*[ starts-with( @id, "comment-" ) ]' );
+			$comments = $this->dom->xpath->query( '//amp-live-list/*[ @items ]/*[ starts-with( @id, "comment-" ) ]' );
 
 			foreach ( $comments as $comment ) {
 				$this->add_amp_live_list_comment_attributes( $comment );
@@ -110,7 +111,7 @@ protected function process_comment_form( $comment_form ) {
 			}
 		}
 
-		$amp_bind_attr_format = AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX . '%s';
+		$amp_bind_attr_format = Document::AMP_BIND_DATA_ATTR_PREFIX . '%s';
 		foreach ( $form_fields as $name => $form_field ) {
 			foreach ( $form_field as $element ) {
 
diff --git a/includes/sanitizers/class-amp-core-theme-sanitizer.php b/includes/sanitizers/class-amp-core-theme-sanitizer.php
index 4146fc98750..43c74bc9bff 100644
--- a/includes/sanitizers/class-amp-core-theme-sanitizer.php
+++ b/includes/sanitizers/class-amp-core-theme-sanitizer.php
@@ -6,6 +6,8 @@
  * @since 1.0
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_Core_Theme_Sanitizer
  *
@@ -28,22 +30,6 @@ class AMP_Core_Theme_Sanitizer extends AMP_Base_Sanitizer {
 	 */
 	protected $args;
 
-	/**
-	 * Body element.
-	 *
-	 * @since 1.0
-	 * @var DOMElement
-	 */
-	protected $body;
-
-	/**
-	 * XPath.
-	 *
-	 * @since 1.0
-	 * @var DOMXPath
-	 */
-	protected $xpath;
-
 	/**
 	 * Array of themes that are supported.
 	 *
@@ -552,13 +538,6 @@ static function( $html ) {
 	 * @since 1.0
 	 */
 	public function sanitize() {
-		$this->body = $this->dom->getElementsByTagName( 'body' )->item( 0 );
-		if ( ! $this->body ) {
-			return;
-		}
-
-		$this->xpath = new DOMXPath( $this->dom );
-
 		$theme_features = self::get_theme_features( $this->args, false );
 		foreach ( $theme_features as $theme_feature => $feature_args ) {
 			if ( method_exists( $this, $theme_feature ) ) {
@@ -613,7 +592,7 @@ public static function remove_actions( $actions = [] ) {
 	 */
 	public function add_smooth_scrolling( $link_xpaths ) {
 		foreach ( $link_xpaths as $link_xpath ) {
-			foreach ( $this->xpath->query( $link_xpath ) as $link ) {
+			foreach ( $this->dom->xpath->query( $link_xpath ) as $link ) {
 				if ( $link instanceof DOMElement && preg_match( '/#(.+)/', $link->getAttribute( 'href' ), $matches ) ) {
 					$link->setAttribute( 'on', sprintf( 'tap:%s.scrollTo(duration=600)', $matches[1] ) );
 
@@ -942,7 +921,7 @@ public function add_twentyseventeen_sticky_nav_menu() {
 		 * @var DOMElement $navigation_top
 		 * @var DOMElement $navigation_top_fixed
 		 */
-		$navigation_top = $this->xpath->query( '//header[ @id = "masthead" ]//div[ contains( @class, "navigation-top" ) ]' )->item( 0 );
+		$navigation_top = $this->dom->xpath->query( '//header[ @id = "masthead" ]//div[ contains( @class, "navigation-top" ) ]' )->item( 0 );
 		if ( ! $navigation_top ) {
 			return;
 		}
@@ -956,7 +935,7 @@ public function add_twentyseventeen_sticky_nav_menu() {
 		}
 
 		$navigation_top->parentNode->insertBefore( $navigation_top_fixed, $navigation_top->nextSibling );
-		foreach ( $this->xpath->query( './/*[ @id ]', $navigation_top_fixed ) as $element ) {
+		foreach ( $this->dom->xpath->query( './/*[ @id ]', $navigation_top_fixed ) as $element ) {
 			$element->setAttribute( 'id', $element->getAttribute( 'id' ) . '-fixed' );
 		}
 
@@ -1017,7 +996,7 @@ public function add_twentyseventeen_sticky_nav_menu() {
 			$position_script->setAttribute( 'type', 'application/json' );
 			$position_script->appendChild( $this->dom->createTextNode( wp_json_encode( $animation ) ) );
 			$amp_animation->appendChild( $position_script );
-			$this->body->appendChild( $amp_animation );
+			$this->dom->body->appendChild( $amp_animation );
 		}
 	}
 
@@ -1398,7 +1377,7 @@ public function add_twentyfourteen_slider_carousel() {
 			return;
 		}
 
-		$featured_content_inner = $this->xpath->query( './div[ @class = "featured-content-inner" ]', $featured_content )->item( 0 );
+		$featured_content_inner = $this->dom->xpath->query( './div[ @class = "featured-content-inner" ]', $featured_content )->item( 0 );
 		if ( ! $featured_content_inner ) {
 			return;
 		}
@@ -1419,12 +1398,12 @@ public function add_twentyfourteen_slider_carousel() {
 		$amp_carousel_desktop_id = 'twentyFourteenSliderDesktop';
 		$amp_carousel_mobile_id  = 'twentyFourteenSliderMobile';
 		$amp_carousel_attributes = [
-			'layout' => 'responsive',
-			'on'     => "slideChange:AMP.setState( { $selected_slide_state_id: event.index } )",
-			'width'  => '100',
-			'type'   => 'slides',
-			'loop'   => '',
-			AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX . 'slide' => $selected_slide_state_id,
+			'layout'                                      => 'responsive',
+			'on'                                          => "slideChange:AMP.setState( { $selected_slide_state_id: event.index } )",
+			'width'                                       => '100',
+			'type'                                        => 'slides',
+			'loop'                                        => '',
+			Document::AMP_BIND_DATA_ATTR_PREFIX . 'slide' => $selected_slide_state_id,
 		];
 		$amp_carousel_desktop    = AMP_DOM_Utils::create_node(
 			$this->dom,
@@ -1472,7 +1451,7 @@ public function add_twentyfourteen_slider_carousel() {
 				$li->setAttribute( 'selected', '' );
 				$a->setAttribute( 'class', 'slider-active' );
 			}
-			$a->setAttribute( AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX . 'class', "$selected_slide_state_id == $i ? 'slider-active' : ''" );
+			$a->setAttribute( Document::AMP_BIND_DATA_ATTR_PREFIX . 'class', "$selected_slide_state_id == $i ? 'slider-active' : ''" );
 			$a->setAttribute( 'role', 'button' );
 			$a->setAttribute( 'on', "tap:AMP.setState( { $selected_slide_state_id: $i } )" );
 			$li->setAttribute( 'option', (string) $i );
@@ -1490,8 +1469,8 @@ public function add_twentyfourteen_slider_carousel() {
 	 * @link https://github.com/WordPress/wordpress-develop/blob/fc2a8f0e11316d066a686995b8578d82cd5546cf/src/wp-content/themes/twentyfourteen/js/functions.js#L69-L87
 	 */
 	public function add_twentyfourteen_search() {
-		$search_toggle_div  = $this->xpath->query( '//div[ contains( @class, "search-toggle" ) ]' )->item( 0 );
-		$search_toggle_link = $this->xpath->query( './a', $search_toggle_div )->item( 0 );
+		$search_toggle_div  = $this->dom->xpath->query( '//div[ contains( @class, "search-toggle" ) ]' )->item( 0 );
+		$search_toggle_link = $this->dom->xpath->query( './a', $search_toggle_div )->item( 0 );
 		$search_container   = $this->dom->getElementById( 'search-container' );
 		if ( ! $search_toggle_div || ! $search_toggle_link || ! $search_container ) {
 			return;
@@ -1510,7 +1489,7 @@ public function add_twentyfourteen_search() {
 
 		// Update AMP state to show the search bar and focus on search input when tapping on the search button.
 		$search_input_id = 'twentyfourteen_search_input';
-		$search_input_el = $this->xpath->query( './/input[ @name = "s" ]', $search_container )->item( 0 );
+		$search_input_el = $this->dom->xpath->query( './/input[ @name = "s" ]', $search_container )->item( 0 );
 		$search_toggle_link->removeAttribute( 'href' );
 		$on = "tap:AMP.setState( { $hidden_state_id: ! $hidden_state_id } )";
 		if ( $search_input_el ) {
@@ -1523,9 +1502,9 @@ public function add_twentyfourteen_search() {
 
 		// Set visibility and aria-expanded based of the link based on whether the search bar is expanded.
 		$search_toggle_link->setAttribute( 'aria-expanded', wp_json_encode( $hidden ) );
-		$search_toggle_link->setAttribute( AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX . 'aria-expanded', "$hidden_state_id ? 'false' : 'true'" );
-		$search_toggle_div->setAttribute( AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX . 'class', "$hidden_state_id ? 'search-toggle' : 'search-toggle active'" );
-		$search_container->setAttribute( AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX . 'class', "$hidden_state_id ? 'search-box-wrapper hide' : 'search-box-wrapper'" );
+		$search_toggle_link->setAttribute( Document::AMP_BIND_DATA_ATTR_PREFIX . 'aria-expanded', "$hidden_state_id ? 'false' : 'true'" );
+		$search_toggle_div->setAttribute( Document::AMP_BIND_DATA_ATTR_PREFIX . 'class', "$hidden_state_id ? 'search-toggle' : 'search-toggle active'" );
+		$search_container->setAttribute( Document::AMP_BIND_DATA_ATTR_PREFIX . 'class', "$hidden_state_id ? 'search-box-wrapper hide' : 'search-box-wrapper'" );
 	}
 
 	/**
@@ -1548,13 +1527,13 @@ public function wrap_modal_in_lightbox( $args = [] ) {
 		}
 
 		$modal_id           = $args['modal_id'];
-		$modal_content_node = $this->xpath->query( $args['modal_content_xpath'] )->item( 0 );
+		$modal_content_node = $this->dom->xpath->query( $args['modal_content_xpath'] )->item( 0 );
 
 		if ( ! is_string( $modal_id ) || ! $modal_content_node instanceof DOMElement ) {
 			return;
 		}
 
-		$body_id = AMP_DOM_Utils::get_element_id( $this->get_body_node(), 'body' );
+		$body_id = AMP_DOM_Utils::get_element_id( $this->dom->body, 'body' );
 
 		$open_xpaths  = isset( $args['open_button_xpath'] ) ? $args['open_button_xpath'] : [];
 		$close_xpaths = isset( $args['close_button_xpath'] ) ? $args['close_button_xpath'] : [];
@@ -1577,7 +1556,7 @@ public function wrap_modal_in_lightbox( $args = [] ) {
 		// retrieve a single result.
 		foreach ( $modal_actions as $modal_action => $toggle_xpaths ) {
 			foreach ( $toggle_xpaths as $toggle_xpath ) {
-				foreach ( $this->xpath->query( $toggle_xpath ) as $toggle_node ) {
+				foreach ( $this->dom->xpath->query( $toggle_xpath ) as $toggle_node ) {
 					if ( $toggle_node instanceof DOMElement ) {
 						AMP_DOM_Utils::add_amp_action( $toggle_node, 'tap', $modal_action );
 					}
@@ -1599,7 +1578,7 @@ public function wrap_modal_in_lightbox( $args = [] ) {
 		$amp_lightbox->setAttribute( 'animate-in', isset( $args['animate_in'] ) ? $args['animate_in'] : 'fade-in' );
 		$amp_lightbox->setAttribute( 'scrollable', isset( $args['scrollable'] ) ? $args['scrollable'] : true );
 
-		$amp_lightbox_inner_content = $this->xpath->query( ".//*[ @class and contains( concat( ' ', normalize-space( @class ), ' ' ), ' modal-inner ' ) ]", $modal_content_node )->item( 0 );
+		$amp_lightbox_inner_content = $this->dom->xpath->query( ".//*[ @class and contains( concat( ' ', normalize-space( @class ), ' ' ), ' modal-inner ' ) ]", $modal_content_node )->item( 0 );
 		foreach ( [ $amp_lightbox, $amp_lightbox_inner_content ] as $event_element ) {
 			$event_element->setAttribute( 'role', $this->guess_modal_role( $modal_content_node ) );
 			// Setting tabindex to -1 (not reachable) as keyboard focus is handled through toggles.
@@ -1635,13 +1614,13 @@ public function wrap_modal_in_lightbox( $args = [] ) {
 	}
 
 	/**
-	 * Add generic modal interactivity compat for the Twentytwenty theme.
+	 * Add generic modal interactivity compat for the Twenty Twenty theme.
 	 *
 	 * Modals implemented in JS will be transformed into <amp-lightbox> equivalents,
 	 * with the tap actions being attached to their associated toggles.
 	 */
 	public function add_twentytwenty_modals() {
-		$modals = $this->xpath->query( "//*[ @class and contains( concat( ' ', normalize-space( @class ), ' ' ), ' cover-modal ' ) ]" );
+		$modals = $this->dom->xpath->query( "//*[ @class and contains( concat( ' ', normalize-space( @class ), ' ' ), ' cover-modal ' ) ]" );
 
 		if ( false === $modals || 0 === $modals->length ) {
 			return;
@@ -1659,7 +1638,7 @@ public function add_twentytwenty_modals() {
 			}
 
 			$modal_target = $modal->getAttribute( 'data-modal-target-string' );
-			$toggles      = $this->xpath->query( "//*[ @data-toggle-target = '{$modal_target}' ]" );
+			$toggles      = $this->dom->xpath->query( "//*[ @data-toggle-target = '{$modal_target}' ]" );
 
 			$open_button_xpaths  = [];
 			$close_button_xpaths = [];
@@ -1718,8 +1697,8 @@ public function add_twentytwenty_modals() {
 	 * with <amp-state> components storing the CSS classes to set.
 	 */
 	public function add_twentytwenty_toggles() {
-		$toggles = $this->xpath->query( '//*[ @data-toggle-target ]' );
-		$body_id = AMP_DOM_Utils::get_element_id( $this->get_body_node(), 'body' );
+		$toggles = $this->dom->xpath->query( '//*[ @data-toggle-target ]' );
+		$body_id = AMP_DOM_Utils::get_element_id( $this->dom->body, 'body' );
 
 		if ( false === $toggles || 0 === $toggles->length ) {
 			return;
@@ -1743,7 +1722,7 @@ public function add_twentytwenty_toggles() {
 					continue;
 				}
 
-				$target_nodes = $this->xpath->query( $target_xpath, $toggle );
+				$target_nodes = $this->dom->xpath->query( $target_xpath, $toggle );
 				if ( false === $target_nodes || 0 === count( $target_nodes ) ) {
 					continue;
 				}
@@ -1775,7 +1754,7 @@ public function add_twentytwenty_toggles() {
 				AMP_DOM_Utils::add_amp_action( $toggle, 'tap', "{$toggle_id}.toggleClass(class='active')" );
 			} else {
 				// If not, toggle all toggles with this toggle target.
-				$target_toggles = $this->xpath->query( "//*[ @data-toggle-target = '{$toggle_target}' ]" );
+				$target_toggles = $this->dom->xpath->query( "//*[ @data-toggle-target = '{$toggle_target}' ]" );
 				foreach ( $target_toggles as $target_toggle ) {
 					if ( AMP_DOM_Utils::has_class( $target_toggle, 'close-nav-toggle' ) ) {
 						// Skip adding the 'active' class on the "Close" button in the primary nav menu.
@@ -1797,7 +1776,7 @@ public function add_twentytwenty_toggles() {
 
 				if ( ! empty( $focus_selector ) ) {
 					$focus_xpath   = $this->xpath_from_css_selector( $focus_selector );
-					$focus_element = $this->xpath->query( $focus_xpath )->item( 0 );
+					$focus_element = $this->dom->xpath->query( $focus_xpath )->item( 0 );
 
 					if ( $focus_element instanceof DOMElement ) {
 						$focus_element_id = AMP_DOM_Utils::get_element_id( $focus_element );
@@ -1825,7 +1804,7 @@ protected function get_closest_submenu( DOMElement $element ) {
 			}
 		}
 
-		$sub_menu = $this->xpath->query( ".//*[ @class and contains( concat( ' ', normalize-space( @class ), ' ' ), ' sub-menu ' ) ]", $menu_item )->item( 0 );
+		$sub_menu = $this->dom->xpath->query( ".//*[ @class and contains( concat( ' ', normalize-space( @class ), ' ' ), ' sub-menu ' ) ]", $menu_item )->item( 0 );
 
 		if ( ! $sub_menu instanceof DOMElement ) {
 			return $element;
@@ -1838,10 +1817,10 @@ protected function get_closest_submenu( DOMElement $element ) {
 	 * Automatically open the submenus related to the current page in the menu modal.
 	 */
 	public function add_twentytwenty_current_page_awareness() {
-		$page_ancestors = $this->xpath->query( "//li[ @class and contains( concat( ' ', normalize-space( @class ), ' ' ), ' current_page_ancestor ' ) ]" );
+		$page_ancestors = $this->dom->xpath->query( "//li[ @class and contains( concat( ' ', normalize-space( @class ), ' ' ), ' current_page_ancestor ' ) ]" );
 		foreach ( $page_ancestors as $page_ancestor ) {
-			$toggle   = $this->xpath->query( "./div/button[ @class and contains( concat( ' ', normalize-space( @class ), ' ' ), ' sub-menu-toggle ' ) ]", $page_ancestor )->item( 0 );
-			$children = $this->xpath->query( "./ul[ @class and contains( concat( ' ', normalize-space( @class ), ' ' ), ' children ' ) ]", $page_ancestor )->item( 0 );
+			$toggle   = $this->dom->xpath->query( "./div/button[ @class and contains( concat( ' ', normalize-space( @class ), ' ' ), ' sub-menu-toggle ' ) ]", $page_ancestor )->item( 0 );
+			$children = $this->dom->xpath->query( "./ul[ @class and contains( concat( ' ', normalize-space( @class ), ' ' ), ' children ' ) ]", $page_ancestor )->item( 0 );
 			foreach ( [ $toggle, $children ] as $element ) {
 				if ( ! $element instanceof DOMElement ) {
 					continue;
diff --git a/includes/sanitizers/class-amp-dev-mode-sanitizer.php b/includes/sanitizers/class-amp-dev-mode-sanitizer.php
index 6dae710f724..f68fc681755 100644
--- a/includes/sanitizers/class-amp-dev-mode-sanitizer.php
+++ b/includes/sanitizers/class-amp-dev-mode-sanitizer.php
@@ -32,10 +32,9 @@ final class AMP_Dev_Mode_Sanitizer extends AMP_Base_Sanitizer {
 	public function sanitize() {
 		$this->dom->documentElement->setAttribute( AMP_Rule_Spec::DEV_MODE_ATTRIBUTE, '' );
 
-		$xpath          = new DOMXPath( $this->dom );
 		$element_xpaths = ! empty( $this->args['element_xpaths'] ) ? $this->args['element_xpaths'] : [];
 		foreach ( $element_xpaths as $element_xpath ) {
-			foreach ( $xpath->query( $element_xpath ) as $node ) {
+			foreach ( $this->dom->xpath->query( $element_xpath ) as $node ) {
 				if ( $node instanceof DOMElement ) {
 					$node->setAttribute( AMP_Rule_Spec::DEV_MODE_ATTRIBUTE, '' );
 				}
diff --git a/includes/sanitizers/class-amp-embed-sanitizer.php b/includes/sanitizers/class-amp-embed-sanitizer.php
index d9f9690d6b2..2fadd8849ae 100644
--- a/includes/sanitizers/class-amp-embed-sanitizer.php
+++ b/includes/sanitizers/class-amp-embed-sanitizer.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_Embed_Sanitizer
  *
@@ -22,8 +24,8 @@ class AMP_Embed_Sanitizer extends AMP_Base_Sanitizer {
 	/**
 	 * AMP_Embed_Sanitizer constructor.
 	 *
-	 * @param DOMDocument $dom  DOM.
-	 * @param array       $args Args.
+	 * @param Document $dom  DOM.
+	 * @param array    $args Args.
 	 */
 	public function __construct( $dom, $args = [] ) {
 		parent::__construct( $dom, $args );
diff --git a/includes/sanitizers/class-amp-form-sanitizer.php b/includes/sanitizers/class-amp-form-sanitizer.php
index e9957ae1304..5e2f48a2c90 100644
--- a/includes/sanitizers/class-amp-form-sanitizer.php
+++ b/includes/sanitizers/class-amp-form-sanitizer.php
@@ -25,7 +25,7 @@ class AMP_Form_Sanitizer extends AMP_Base_Sanitizer {
 	public static $tag = 'form';
 
 	/**
-	 * Sanitize the <form> elements from the HTML contained in this instance's DOMDocument.
+	 * Sanitize the <form> elements from the HTML contained in this instance's Dom\Document.
 	 *
 	 * @link https://www.ampproject.org/docs/reference/components/amp-form
 	 * @since 0.7
diff --git a/includes/sanitizers/class-amp-gallery-block-sanitizer.php b/includes/sanitizers/class-amp-gallery-block-sanitizer.php
index 30c8bff6fe7..8f34997d441 100644
--- a/includes/sanitizers/class-amp-gallery-block-sanitizer.php
+++ b/includes/sanitizers/class-amp-gallery-block-sanitizer.php
@@ -5,7 +5,7 @@
  * @package AMP
  */
 
-use Amp\AmpWP\Component\DOMElementList;
+use Amp\AmpWP\Dom\ElementList;
 use Amp\AmpWP\Component\Carousel;
 
 /**
@@ -58,7 +58,6 @@ class AMP_Gallery_Block_Sanitizer extends AMP_Base_Sanitizer {
 	 * @since 0.2
 	 */
 	public function sanitize() {
-		$xpath       = new DOMXPath( $this->dom );
 		$class_query = 'contains( concat( " ", normalize-space( @class ), " " ), " wp-block-gallery " )';
 		$expr        = sprintf(
 			'//ul[ %s ]',
@@ -70,7 +69,7 @@ public function sanitize() {
 				]
 			)
 		);
-		$query       = $xpath->query( $expr );
+		$query       = $this->dom->xpath->query( $expr );
 
 		$nodes = [];
 		foreach ( $query as $node ) {
@@ -106,7 +105,7 @@ public function sanitize() {
 				continue;
 			}
 
-			$images = new DOMElementList();
+			$images = new ElementList();
 
 			// If it's not AMP lightbox, look for links first.
 			if ( ! $is_amp_lightbox ) {
diff --git a/includes/sanitizers/class-amp-iframe-sanitizer.php b/includes/sanitizers/class-amp-iframe-sanitizer.php
index e606098a141..bf7d329125e 100644
--- a/includes/sanitizers/class-amp-iframe-sanitizer.php
+++ b/includes/sanitizers/class-amp-iframe-sanitizer.php
@@ -64,7 +64,7 @@ public function get_selector_conversion_mapping() {
 	}
 
 	/**
-	 * Sanitize the <iframe> elements from the HTML contained in this instance's DOMDocument.
+	 * Sanitize the <iframe> elements from the HTML contained in this instance's Dom\Document.
 	 *
 	 * @since 0.2
 	 */
diff --git a/includes/sanitizers/class-amp-img-sanitizer.php b/includes/sanitizers/class-amp-img-sanitizer.php
index 43c54ef00a0..a1ff0847fec 100644
--- a/includes/sanitizers/class-amp-img-sanitizer.php
+++ b/includes/sanitizers/class-amp-img-sanitizer.php
@@ -71,7 +71,7 @@ public function get_selector_conversion_mapping() {
 	}
 
 	/**
-	 * Sanitize the <img> elements from the HTML contained in this instance's DOMDocument.
+	 * Sanitize the <img> elements from the HTML contained in this instance's Dom\Document.
 	 *
 	 * @since 0.2
 	 */
diff --git a/includes/sanitizers/class-amp-link-sanitizer.php b/includes/sanitizers/class-amp-link-sanitizer.php
index 490184175d3..59dd63a5327 100644
--- a/includes/sanitizers/class-amp-link-sanitizer.php
+++ b/includes/sanitizers/class-amp-link-sanitizer.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_Link_Sanitizer.
  *
@@ -63,10 +65,10 @@ class AMP_Link_Sanitizer extends AMP_Base_Sanitizer {
 	/**
 	 * Sanitizer constructor.
 	 *
-	 * @param DOMDocument $dom  Document.
-	 * @param array       $args Args.
+	 * @param Document $dom  Document.
+	 * @param array    $args Args.
 	 */
-	public function __construct( DOMDocument $dom, array $args = [] ) {
+	public function __construct( $dom, array $args = [] ) {
 		if ( ! isset( $args['meta_content'] ) ) {
 			$args['meta_content'] = self::DEFAULT_META_CONTENT;
 		}
@@ -96,14 +98,13 @@ public function sanitize() {
 	 * @return DOMElement|null The added meta element if successful.
 	 */
 	public function add_meta_tag( $content = self::DEFAULT_META_CONTENT ) {
-		$head = $this->dom->documentElement->getElementsByTagName( 'head' )->item( 0 );
-		if ( ! $head || ! $content ) {
+		if ( ! $content ) {
 			return null;
 		}
 		$meta = $this->dom->createElement( 'meta' );
 		$meta->setAttribute( 'name', 'amp-to-amp-navigation' );
 		$meta->setAttribute( 'content', $content );
-		$head->appendChild( $meta );
+		$this->dom->head->appendChild( $meta );
 		return $meta;
 	}
 
@@ -116,7 +117,6 @@ public function process_links() {
 		 *
 		 * @var DOMElement $element
 		 */
-		$xpath = new DOMXPath( $this->dom );
 
 		// Remove admin bar from DOM to prevent mutating it.
 		$admin_bar_container   = $this->dom->getElementById( 'wpadminbar' );
@@ -126,7 +126,7 @@ public function process_links() {
 			$admin_bar_container->parentNode->replaceChild( $admin_bar_placeholder, $admin_bar_container );
 		}
 
-		foreach ( $xpath->query( '//*[ local-name() = "a" or local-name() = "area" ]' ) as $element ) {
+		foreach ( $this->dom->xpath->query( '//*[ local-name() = "a" or local-name() = "area" ]' ) as $element ) {
 			if ( ! $element->hasAttribute( 'href' ) ) {
 				continue;
 			}
@@ -147,7 +147,7 @@ public function process_links() {
 			}
 		}
 
-		foreach ( $xpath->query( '//form[ @action and translate( @method, "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "abcdefghijklmnopqrstuvwxyz") = "get" ]' ) as $element ) {
+		foreach ( $this->dom->xpath->query( '//form[ @action and translate( @method, "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "abcdefghijklmnopqrstuvwxyz") = "get" ]' ) as $element ) {
 			if ( $this->is_frontend_url( $element->getAttribute( 'action' ) ) ) {
 				$input = $this->dom->createElement( 'input' );
 				$input->setAttribute( 'name', amp_get_slug() );
diff --git a/includes/sanitizers/class-amp-meta-sanitizer.php b/includes/sanitizers/class-amp-meta-sanitizer.php
new file mode 100644
index 00000000000..5628ef3f320
--- /dev/null
+++ b/includes/sanitizers/class-amp-meta-sanitizer.php
@@ -0,0 +1,243 @@
+<?php
+/**
+ * Class AMP_Links_Sanitizer.
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Meta_Sanitizer.
+ *
+ * Sanitizes meta tags found in the header.
+ *
+ * @since 1.5.0
+ */
+class AMP_Meta_Sanitizer extends AMP_Base_Sanitizer {
+
+	/**
+	 * Tag.
+	 *
+	 * @var string HTML <meta> tag to identify and replace with AMP version.
+	 */
+	public static $tag = 'meta';
+
+	/*
+	 * Tags array keys.
+	 */
+	const TAG_CHARSET        = 'charset';
+	const TAG_VIEWPORT       = 'viewport';
+	const TAG_AMP_SCRIPT_SRC = 'amp_script_src';
+	const TAG_OTHER          = 'other';
+
+	/**
+	 * Associative array of DOMElement arrays.
+	 *
+	 * Each key in the root level defines one group of meta tags to process.
+	 *
+	 * @var array $tags {
+	 *     An array of meta tag groupings.
+	 *
+	 *     @type DOMElement[] $charset        Charset meta tag(s).
+	 *     @type DOMElement[] $viewport       Viewport meta tag(s).
+	 *     @type DOMElement[] $amp_script_src <amp-script> source meta tags.
+	 *     @type DOMElement[] $other          Remaining meta tags.
+	 * }
+	 */
+	protected $meta_tags = [
+		self::TAG_CHARSET        => [],
+		self::TAG_VIEWPORT       => [],
+		self::TAG_AMP_SCRIPT_SRC => [],
+		self::TAG_OTHER          => [],
+	];
+
+	/**
+	 * Charset to use for AMP markup.
+	 *
+	 * @var string
+	 */
+	const AMP_CHARSET = 'utf-8';
+
+	/**
+	 * Viewport settings to use for AMP markup.
+	 *
+	 * @var string
+	 */
+	const AMP_VIEWPORT = 'width=device-width';
+
+	/**
+	 * Sanitize.
+	 */
+	public function sanitize() {
+		$elements = $this->dom->getElementsByTagName( static::$tag );
+
+		// Remove all nodes for easy reordering later on.
+		$elements = array_map(
+			static function ( $element ) {
+				return $element->parentNode->removeChild( $element );
+			},
+			iterator_to_array( $elements, false )
+		);
+
+		foreach ( $elements as $element ) {
+			/**
+			 * Meta tag to process.
+			 *
+			 * @var DOMElement $element
+			 */
+			if ( $element->hasAttribute( 'charset' ) ) {
+				$this->meta_tags[ self::TAG_CHARSET ][] = $element;
+			} elseif ( 'viewport' === $element->getAttribute( 'name' ) ) {
+				$this->meta_tags[ self::TAG_VIEWPORT ][] = $element;
+			} elseif ( 'amp-script-src' === $element->getAttribute( 'name' ) ) {
+				$this->meta_tags[ self::TAG_AMP_SCRIPT_SRC ][] = $element;
+			} else {
+				$this->meta_tags[ self::TAG_OTHER ][] = $element;
+			}
+		}
+
+		$this->ensure_charset_is_present();
+
+		$this->ensure_viewport_is_present();
+
+		$this->process_amp_script_meta_tags();
+
+		$this->re_add_meta_tags_in_optimized_order();
+	}
+
+
+	/**
+	 * Always ensure that we have an HTML 5 charset meta tag.
+	 *
+	 * The charset is set to utf-8, which is what AMP requires.
+	 */
+	protected function ensure_charset_is_present() {
+		if ( ! empty( $this->meta_tags[ self::TAG_CHARSET ] ) ) {
+			return;
+		}
+
+		$this->meta_tags[ self::TAG_CHARSET ][] = $this->create_charset_element();
+	}
+
+	/**
+	 * Always ensure we have a viewport tag.
+	 *
+	 * The viewport defaults to 'width=device-width', which is the bare minimum that AMP requires.
+	 */
+	protected function ensure_viewport_is_present() {
+		if ( empty( $this->meta_tags[ self::TAG_VIEWPORT ] ) ) {
+			$this->meta_tags[ self::TAG_VIEWPORT ][] = $this->create_viewport_element( static::AMP_VIEWPORT );
+			return;
+		}
+
+		// Ensure we have the 'width=device-width' setting included.
+		/**
+		 * Viewport tag.
+		 *
+		 * @var DOMElement $viewport_tag
+		 */
+		$viewport_tag      = $this->meta_tags[ self::TAG_VIEWPORT ][0];
+		$viewport_content  = $viewport_tag->getAttribute( 'content' );
+		$viewport_settings = array_map( 'trim', explode( ',', $viewport_content ) );
+		$width_found       = false;
+
+		// @todo The invalid/missing properties should raise validation errors. See <https://github.com/ampproject/amp-wp/pull/3758/files#r348074703>.
+		foreach ( $viewport_settings as $index => $viewport_setting ) {
+			list( $property, $value ) = array_map( 'trim', explode( '=', $viewport_setting ) );
+			if ( 'width' === $property ) {
+				if ( 'device-width' !== $value ) {
+					$viewport_settings[ $index ] = 'width=device-width';
+				}
+				$width_found = true;
+				break;
+			}
+		}
+
+		if ( ! $width_found ) {
+			array_unshift( $viewport_settings, 'width=device-width' );
+		}
+
+		$viewport_tag->setAttribute( 'content', implode( ',', $viewport_settings ) );
+	}
+
+	/**
+	 * Parse and concatenate <amp-script> source meta tags.
+	 */
+	protected function process_amp_script_meta_tags() {
+		if ( empty( $this->meta_tags[ self::TAG_AMP_SCRIPT_SRC ] ) ) {
+			return;
+		}
+
+		$first_meta_amp_script_src = array_shift( $this->meta_tags[ self::TAG_AMP_SCRIPT_SRC ] );
+		$content_values            = [ $first_meta_amp_script_src->getAttribute( 'content' ) ];
+
+		// Merge (and remove) any subsequent meta amp-script-src elements.
+		while ( ! empty( $this->meta_tags[ self::TAG_AMP_SCRIPT_SRC ] ) ) {
+			$meta_amp_script_src = array_shift( $this->meta_tags[ self::TAG_AMP_SCRIPT_SRC ] );
+			$content_values[]    = $meta_amp_script_src->getAttribute( 'content' );
+		}
+
+		$first_meta_amp_script_src->setAttribute( 'content', implode( ' ', $content_values ) );
+
+		$this->meta_tags[ self::TAG_AMP_SCRIPT_SRC ][] = $first_meta_amp_script_src;
+	}
+
+	/**
+	 * Create a new meta tag for the charset value.
+	 *
+	 * @return DOMElement New meta tag with requested charset.
+	 */
+	protected function create_charset_element() {
+		return AMP_DOM_Utils::create_node(
+			$this->dom,
+			'meta',
+			[
+				'charset' => self::AMP_CHARSET,
+			]
+		);
+	}
+
+	/**
+	 * Create a new meta tag for the viewport setting.
+	 *
+	 * @param string $viewport Viewport setting to use.
+	 * @return DOMElement New meta tag with requested viewport setting.
+	 */
+	protected function create_viewport_element( $viewport ) {
+		return AMP_DOM_Utils::create_node(
+			$this->dom,
+			'meta',
+			[
+				'name'    => 'viewport',
+				'content' => $viewport,
+			]
+		);
+	}
+
+	/**
+	 * Re-add the meta tags to the <head> node in the optimized order.
+	 *
+	 * The order is defined by the array entries in $this->meta_tags.
+	 *
+	 * The optimal loading order for AMP pages is documented at:
+	 * https://amp.dev/documentation/guides-and-tutorials/optimize-and-measure/optimize_amp/#optimize-the-amp-runtime-loading
+	 *
+	 * "1. The first tag should be the meta charset tag, followed by any remaining meta tags."
+	 */
+	protected function re_add_meta_tags_in_optimized_order() {
+		/**
+		 * Previous meta tag to append to.
+		 *
+		 * @var DOMElement $previous_meta_tag
+		 */
+		$previous_meta_tag = null;
+		foreach ( $this->meta_tags as $meta_tag_group ) {
+			foreach ( $meta_tag_group as $meta_tag ) {
+				if ( $previous_meta_tag ) {
+					$previous_meta_tag = $this->dom->head->insertBefore( $meta_tag, $previous_meta_tag->nextSibling );
+				} else {
+					$previous_meta_tag = $this->dom->head->insertBefore( $meta_tag, $this->dom->head->firstChild );
+				}
+			}
+		}
+	}
+}
diff --git a/includes/sanitizers/class-amp-nav-menu-dropdown-sanitizer.php b/includes/sanitizers/class-amp-nav-menu-dropdown-sanitizer.php
index e686951c65f..bd473e582b5 100644
--- a/includes/sanitizers/class-amp-nav-menu-dropdown-sanitizer.php
+++ b/includes/sanitizers/class-amp-nav-menu-dropdown-sanitizer.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_Nav_Menu_Dropdown_Sanitizer
  *
@@ -34,8 +36,8 @@ class AMP_Nav_Menu_Dropdown_Sanitizer extends AMP_Base_Sanitizer {
 	 *
 	 * @since 1.1.0
 	 *
-	 * @param DOMDocument $dom  DOM.
-	 * @param array       $args Args.
+	 * @param Document $dom  DOM.
+	 * @param array    $args Args.
 	 */
 	public function __construct( $dom, $args = [] ) {
 		parent::__construct( $dom, $args );
diff --git a/includes/sanitizers/class-amp-nav-menu-toggle-sanitizer.php b/includes/sanitizers/class-amp-nav-menu-toggle-sanitizer.php
index aaa35a62b59..ccebe82187c 100644
--- a/includes/sanitizers/class-amp-nav-menu-toggle-sanitizer.php
+++ b/includes/sanitizers/class-amp-nav-menu-toggle-sanitizer.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_Nav_Menu_Toggle_Sanitizer
  *
@@ -30,21 +32,13 @@ class AMP_Nav_Menu_Toggle_Sanitizer extends AMP_Base_Sanitizer {
 		'nav_menu_toggle_state_id'   => 'navMenuToggledOn',
 	];
 
-	/**
-	 * XPath.
-	 *
-	 * @since 1.1.0
-	 * @var DOMXPath
-	 */
-	protected $xpath;
-
 	/**
 	 * AMP_Nav_Menu_Toggle_Sanitizer constructor.
 	 *
 	 * @since 1.1.0
 	 *
-	 * @param DOMDocument $dom  DOM.
-	 * @param array       $args Args.
+	 * @param Document $dom  DOM.
+	 * @param array    $args Args.
 	 */
 	public function __construct( $dom, $args = [] ) {
 		parent::__construct( $dom, $args );
@@ -61,8 +55,6 @@ public function __construct( $dom, $args = [] ) {
 	 * @since 1.1.0
 	 */
 	public function sanitize() {
-		$this->xpath = new DOMXPath( $this->dom );
-
 		$nav_el    = $this->get_nav_container();
 		$button_el = $this->get_menu_button();
 
@@ -85,7 +77,7 @@ public function sanitize() {
 
 		if ( ! empty( $this->args['nav_container_toggle_class'] ) ) {
 			$nav_el->setAttribute(
-				AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX . 'class',
+				Document::AMP_BIND_DATA_ATTR_PREFIX . 'class',
 				sprintf(
 					"%s + ( $state_id ? %s : '' )",
 					wp_json_encode( $nav_el->getAttribute( 'class' ) ),
@@ -103,8 +95,7 @@ public function sanitize() {
 		if ( 'body' === $nav_el->nodeName ) {
 			$nav_el->insertBefore( $state_el, $nav_el->firstChild );
 		} elseif ( $nav_el === $this->dom->documentElement ) {
-			$body = $this->dom->getElementsByTagName( 'body' )->item( 0 );
-			$body->insertBefore( $state_el, $body->firstChild );
+			$this->dom->body->insertBefore( $state_el, $this->dom->body->firstChild );
 		} else {
 			$nav_el->parentNode->insertBefore( $state_el, $nav_el );
 		}
@@ -112,10 +103,10 @@ public function sanitize() {
 		$button_on = sprintf( "tap:AMP.setState({ $state_id: ! $state_id })" );
 		$button_el->setAttribute( 'on', $button_on );
 		$button_el->setAttribute( 'aria-expanded', 'false' );
-		$button_el->setAttribute( AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX . 'aria-expanded', "$state_id ? 'true' : 'false'" );
+		$button_el->setAttribute( Document::AMP_BIND_DATA_ATTR_PREFIX . 'aria-expanded', "$state_id ? 'true' : 'false'" );
 		if ( ! empty( $this->args['menu_button_toggle_class'] ) ) {
 			$button_el->setAttribute(
-				AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX . 'class',
+				Document::AMP_BIND_DATA_ATTR_PREFIX . 'class',
 				sprintf( "%s + ( $state_id ? %s : '' )", wp_json_encode( $button_el->getAttribute( 'class' ) ), wp_json_encode( ' ' . $this->args['menu_button_toggle_class'] ) )
 			);
 		}
@@ -134,7 +125,10 @@ protected function get_nav_container() {
 		}
 
 		if ( ! empty( $this->args['nav_container_xpath'] ) ) {
-			return $this->xpath->query( $this->args['nav_container_xpath'] )->item( 0 );
+			$node = $this->dom->xpath->query( $this->args['nav_container_xpath'] )->item( 0 );
+			if ( $node instanceof DOMElement ) {
+				return $node;
+			}
 		}
 
 		return null;
@@ -153,7 +147,10 @@ protected function get_menu_button() {
 		}
 
 		if ( ! empty( $this->args['menu_button_xpath'] ) ) {
-			return $this->xpath->query( $this->args['menu_button_xpath'] )->item( 0 );
+			$node = $this->dom->xpath->query( $this->args['menu_button_xpath'] )->item( 0 );
+			if ( $node instanceof DOMElement ) {
+				return $node;
+			}
 		}
 
 		return null;
diff --git a/includes/sanitizers/class-amp-o2-player-sanitizer.php b/includes/sanitizers/class-amp-o2-player-sanitizer.php
index f21b8ff1cfd..2669b5dfc76 100644
--- a/includes/sanitizers/class-amp-o2-player-sanitizer.php
+++ b/includes/sanitizers/class-amp-o2-player-sanitizer.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_O2_Player_Sanitizer
  *
@@ -54,24 +56,17 @@ class AMP_O2_Player_Sanitizer extends AMP_Base_Sanitizer {
 	private static $width = '480';
 
 	/**
-	 * Sanitize the O2 Player elements from the HTML contained in this instance's DOMDocument.
+	 * Sanitize the O2 Player elements from the HTML contained in this instance's Dom\Document.
 	 *
 	 * @since 1.0
 	 */
 	public function sanitize() {
-		/**
-		 * XPath.
-		 *
-		 * @var DOMXPath $xpath
-		 */
-		$xpath = new DOMXPath( $this->dom );
-
 		/**
 		 * Node list.
 		 *
 		 * @var DOMNodeList $nodes
 		 */
-		$nodes     = $xpath->query( self::$xpath_selector );
+		$nodes     = $this->dom->xpath->query( self::$xpath_selector );
 		$num_nodes = $nodes->length;
 
 		if ( 0 === $num_nodes ) {
@@ -80,8 +75,9 @@ public function sanitize() {
 
 		for ( $i = $num_nodes - 1; $i >= 0; $i-- ) {
 			$node = $nodes->item( $i );
-
-			$this->create_amp_o2_player( $this->dom, $node );
+			if ( $node instanceof DOMElement ) {
+				$this->create_amp_o2_player( $this->dom, $node );
+			}
 		}
 
 	}
@@ -90,10 +86,10 @@ public function sanitize() {
 	 * Replaces node with amp-o2-player
 	 *
 	 * @since 1.0
-	 * @param DOMDocument $dom  The HTML Document.
-	 * @param DOMElement  $node The DOMNode to adjust and replace.
+	 * @param Document   $dom  The HTML Document.
+	 * @param DOMElement $node The DOMNode to adjust and replace.
 	 */
-	private function create_amp_o2_player( $dom, $node ) {
+	private function create_amp_o2_player( Document $dom, DOMElement $node ) {
 		$o2_attributes = $this->get_o2_player_attributes( $node->getAttribute( 'src' ) );
 
 		if ( ! empty( $o2_attributes ) ) {
diff --git a/includes/sanitizers/class-amp-playbuzz-sanitizer.php b/includes/sanitizers/class-amp-playbuzz-sanitizer.php
index ecf3c419a48..34d77a594c5 100644
--- a/includes/sanitizers/class-amp-playbuzz-sanitizer.php
+++ b/includes/sanitizers/class-amp-playbuzz-sanitizer.php
@@ -52,7 +52,7 @@ public function get_selector_conversion_mapping() {
 	}
 
 	/**
-	 * Sanitize the Playbuzz elements from the HTML contained in this instance's DOMDocument.
+	 * Sanitize the Playbuzz elements from the HTML contained in this instance's Dom\Document.
 	 *
 	 * @since 0.2
 	 */
diff --git a/includes/sanitizers/class-amp-script-sanitizer.php b/includes/sanitizers/class-amp-script-sanitizer.php
index a737ee38b1a..39fd377c879 100644
--- a/includes/sanitizers/class-amp-script-sanitizer.php
+++ b/includes/sanitizers/class-amp-script-sanitizer.php
@@ -50,15 +50,14 @@ public function sanitize() {
 			$fragment->appendChild( $this->dom->createComment( 'noscript' ) );
 			while ( $noscript->firstChild ) {
 				if ( $is_inside_head_el && ! $must_move_to_body ) {
-					$must_move_to_body = ! AMP_DOM_Utils::is_valid_head_node( $noscript->firstChild );
+					$must_move_to_body = ! $this->dom->is_valid_head_node( $noscript->firstChild );
 				}
 				$fragment->appendChild( $noscript->firstChild );
 			}
 			$fragment->appendChild( $this->dom->createComment( '/noscript' ) );
 
 			if ( $must_move_to_body ) {
-				$body = $this->dom->getElementsByTagName( 'body' )->item( 0 );
-				$body->insertBefore( $fragment, $body->firstChild );
+				$this->dom->body->insertBefore( $fragment, $this->dom->body->firstChild );
 				$noscript->parentNode->removeChild( $noscript );
 			} else {
 				$noscript->parentNode->replaceChild( $fragment, $noscript );
diff --git a/includes/sanitizers/class-amp-story-export-sanitizer.php b/includes/sanitizers/class-amp-story-export-sanitizer.php
index 090a4a28305..d0fcac04342 100644
--- a/includes/sanitizers/class-amp-story-export-sanitizer.php
+++ b/includes/sanitizers/class-amp-story-export-sanitizer.php
@@ -66,8 +66,7 @@ public function sanitize() {
 			]
 		);
 
-		$xpath           = new DOMXPath( $this->dom );
-		$schema_org_meta = $xpath->query( '//head/script[@type="application/ld+json"]' )->item( 0 );
+		$schema_org_meta = $this->dom->xpath->query( '//head/script[@type="application/ld+json"]' )->item( 0 );
 
 		if ( $schema_org_meta && $schema_org_meta->firstChild instanceof DOMText ) {
 			$metadata = json_decode( $schema_org_meta->firstChild->nodeValue );
@@ -123,7 +122,7 @@ public function sanitize() {
 
 		// Add or update Canonical URL in the document head.
 		if ( $this->args['base_url'] && $this->args['canonical_url'] ) {
-			$canonical_link = $xpath->query( '/html/head/link[ @rel = "canonical" ]' )->item( 0 );
+			$canonical_link = $this->dom->xpath->query( '/html/head/link[ @rel = "canonical" ]' )->item( 0 );
 			if ( $canonical_link instanceof DOMElement ) {
 				$canonical_link->setAttribute( 'href', $this->args['canonical_url'] );
 			} else {
@@ -135,7 +134,7 @@ public function sanitize() {
 						'href' => $this->args['canonical_url'],
 					]
 				);
-				$this->dom->getElementsByTagName( 'head' )->item( 0 )->appendChild( $canonical_link );
+				$this->dom->head->appendChild( $canonical_link );
 			}
 		}
 
diff --git a/includes/sanitizers/class-amp-style-sanitizer.php b/includes/sanitizers/class-amp-style-sanitizer.php
index 5cb9ccead3a..9bcc886e3f3 100644
--- a/includes/sanitizers/class-amp-style-sanitizer.php
+++ b/includes/sanitizers/class-amp-style-sanitizer.php
@@ -5,19 +5,20 @@
  * @package AMP
  */
 
-use \Sabberworm\CSS\RuleSet\DeclarationBlock;
-use \Sabberworm\CSS\CSSList\CSSList;
-use \Sabberworm\CSS\Property\Selector;
-use \Sabberworm\CSS\RuleSet\RuleSet;
-use \Sabberworm\CSS\Property\AtRule;
-use \Sabberworm\CSS\Rule\Rule;
-use \Sabberworm\CSS\CSSList\KeyFrame;
-use \Sabberworm\CSS\RuleSet\AtRuleSet;
-use \Sabberworm\CSS\Property\Import;
-use \Sabberworm\CSS\CSSList\AtRuleBlockList;
-use \Sabberworm\CSS\Value\RuleValueList;
-use \Sabberworm\CSS\Value\URL;
-use \Sabberworm\CSS\CSSList\Document;
+use Amp\AmpWP\Dom\Document;
+use Sabberworm\CSS\RuleSet\DeclarationBlock;
+use Sabberworm\CSS\CSSList\CSSList;
+use Sabberworm\CSS\Property\Selector;
+use Sabberworm\CSS\RuleSet\RuleSet;
+use Sabberworm\CSS\Property\AtRule;
+use Sabberworm\CSS\Rule\Rule;
+use Sabberworm\CSS\CSSList\KeyFrame;
+use Sabberworm\CSS\RuleSet\AtRuleSet;
+use Sabberworm\CSS\Property\Import;
+use Sabberworm\CSS\CSSList\AtRuleBlockList;
+use Sabberworm\CSS\Value\RuleValueList;
+use Sabberworm\CSS\Value\URL;
+use Sabberworm\CSS\CSSList\Document as CSSDocument;
 
 /**
  * Class AMP_Style_Sanitizer
@@ -247,14 +248,6 @@ class AMP_Style_Sanitizer extends AMP_Base_Sanitizer {
 	 */
 	private $used_tag_names;
 
-	/**
-	 * XPath.
-	 *
-	 * @since 1.0
-	 * @var DOMXPath
-	 */
-	private $xpath;
-
 	/**
 	 * Amount of time that was spent parsing CSS.
 	 *
@@ -263,13 +256,6 @@ class AMP_Style_Sanitizer extends AMP_Base_Sanitizer {
 	 */
 	private $parse_css_duration = 0.0;
 
-	/**
-	 * THe HEAD element.
-	 *
-	 * @var DOMElement
-	 */
-	private $head;
-
 	/**
 	 * Current node being processed.
 	 *
@@ -396,10 +382,10 @@ public static function has_required_php_css_parser() {
 	 *
 	 * @since 0.7
 	 *
-	 * @param DOMDocument $dom  Represents the HTML document to sanitize.
-	 * @param array       $args Args.
+	 * @param Document $dom  Represents the HTML document to sanitize.
+	 * @param array    $args Args.
 	 */
-	public function __construct( DOMDocument $dom, array $args = [] ) {
+	public function __construct( $dom, array $args = [] ) {
 		parent::__construct( $dom, $args );
 
 		foreach ( AMP_Allowed_Tags_Generated::get_allowed_tag( 'style' ) as $spec_rule ) {
@@ -427,11 +413,10 @@ public function __construct( DOMDocument $dom, array $args = [] ) {
 		}
 		$this->base_url    = untrailingslashit( $guessurl );
 		$this->content_url = WP_CONTENT_URL;
-		$this->xpath       = new DOMXPath( $dom );
 	}
 
 	/**
-	 * Get list of CSS styles in HTML content of DOMDocument ($this->dom).
+	 * Get list of CSS styles in HTML content of Dom\Document ($this->dom).
 	 *
 	 * @since 0.4
 	 * @deprecated As of 1.0, use get_stylesheets().
@@ -486,12 +471,12 @@ private function get_used_class_names() {
 		];
 
 		$classes = ' ';
-		foreach ( $this->xpath->query( '//*/@class' ) as $class_attribute ) {
+		foreach ( $this->dom->xpath->query( '//*/@class' ) as $class_attribute ) {
 			$classes .= ' ' . $class_attribute->nodeValue;
 		}
 
 		// Find all [class] attributes and capture the contents of any single- or double-quoted strings.
-		foreach ( $this->xpath->query( '//*/@' . AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX . 'class' ) as $bound_class_attribute ) {
+		foreach ( $this->dom->xpath->query( '//*/@' . Document::AMP_BIND_DATA_ATTR_PREFIX . 'class' ) as $bound_class_attribute ) {
 			if ( preg_match_all( '/([\'"])([^\1]*?)\1/', $bound_class_attribute->nodeValue, $matches ) ) {
 				$classes .= ' ' . implode( ' ', $matches[2] );
 			}
@@ -503,7 +488,7 @@ private function get_used_class_names() {
 		);
 
 		// Find all instances of the toggleClass() action to prevent the class name from being tree-shaken.
-		foreach ( $this->xpath->query( '//*/@on[ contains( ., "toggleClass" ) ]' ) as $on_attribute ) {
+		foreach ( $this->dom->xpath->query( '//*/@on[ contains( ., "toggleClass" ) ]' ) as $on_attribute ) {
 			if ( preg_match_all( '/\.\s*toggleClass\s*\(\s*class\s*=\s*(([\'"])([^\1]*?)\2|[a-zA-Z0-9_\-]+)/', $on_attribute->nodeValue, $matches ) ) {
 				$class_names = array_merge(
 					$class_names,
@@ -725,7 +710,7 @@ private function has_used_attributes( $attribute_names ) {
 			if ( ! isset( $this->used_attributes[ $attribute_name ] ) ) {
 				$expression = sprintf( '(//@%s)[1]', $attribute_name );
 
-				$this->used_attributes[ $attribute_name ] = ( 0 !== $this->xpath->query( $expression )->length );
+				$this->used_attributes[ $attribute_name ] = ( 0 !== $this->dom->xpath->query( $expression )->length );
 			}
 
 			// Attributes for amp-accordion, see <https://amp.dev/documentation/components/amp-accordion/#styling>.
@@ -791,7 +776,7 @@ public function init( $sanitizers ) {
 	}
 
 	/**
-	 * Sanitize CSS styles within the HTML contained in this instance's DOMDocument.
+	 * Sanitize CSS styles within the HTML contained in this instance's Dom\Document.
 	 *
 	 * @since 0.4
 	 */
@@ -810,19 +795,12 @@ public function sanitize() {
 				null
 		);
 
-		$this->head = $this->dom->getElementsByTagName( 'head' )->item( 0 );
-		if ( ! $this->head ) {
-			$this->head = $this->dom->createElement( 'head' );
-			$this->dom->documentElement->insertBefore( $this->head, $this->dom->documentElement->firstChild );
-		}
-
 		$this->parse_css_duration = 0.0;
 
 		/*
 		 * Note that xpath is used to query the DOM so that the link and style elements will be
 		 * in document order. DOMNode::compareDocumentPosition() is not yet implemented.
 		 */
-		$xpath = $this->xpath;
 
 		$dev_mode_predicate = '';
 		if ( $this->is_document_in_dev_mode() ) {
@@ -835,7 +813,7 @@ public function sanitize() {
 			sprintf( '( self::link and @href and %s = "stylesheet" %s )', sprintf( $lower_case, '@rel' ), $dev_mode_predicate ),
 		];
 
-		foreach ( $xpath->query( '//*[ ' . implode( ' or ', $predicates ) . ' ]' ) as $element ) {
+		foreach ( $this->dom->xpath->query( '//*[ ' . implode( ' or ', $predicates ) . ' ]' ) as $element ) {
 			$elements[] = $element;
 		}
 
@@ -875,13 +853,13 @@ public function sanitize() {
 
 				// If the element is still in the document, it is a font stylesheet; make sure it gets moved to the head as required.
 				if ( $element->parentNode && 'head' !== $element->parentNode->nodeName ) {
-					$this->head->appendChild( $element->parentNode->removeChild( $element ) );
+					$this->dom->head->appendChild( $element->parentNode->removeChild( $element ) );
 				}
 			}
 		}
 
 		$elements = [];
-		foreach ( $xpath->query( "//*[ @style $dev_mode_predicate ]" ) as $element ) {
+		foreach ( $this->dom->xpath->query( "//*[ @style $dev_mode_predicate ]" ) as $element ) {
 			$elements[] = $element;
 		}
 		foreach ( $elements as $element ) {
@@ -1271,7 +1249,7 @@ private function process_link_element( DOMElement $element ) {
 			$needs_preconnect_link = (
 				'https://fonts.googleapis.com/' === substr( $normalized_url, 0, 29 )
 				&&
-				0 === $this->xpath->query( '//link[ @rel = "preconnect" and @crossorigin and starts-with( @href, "https://fonts.gstatic.com" ) ]', $this->head )->length
+				0 === $this->dom->xpath->query( '//link[ @rel = "preconnect" and @crossorigin and starts-with( @href, "https://fonts.gstatic.com" ) ]', $this->dom->head )->length
 			);
 			if ( $needs_preconnect_link ) {
 				$link = AMP_DOM_Utils::create_node(
@@ -1283,7 +1261,7 @@ private function process_link_element( DOMElement $element ) {
 						'crossorigin' => '',
 					]
 				);
-				$this->head->insertBefore( $link ); // Note that \AMP_Theme_Support::ensure_required_markup() will put this in the optimal order.
+				$this->dom->head->insertBefore( $link ); // Note that \AMP_Theme_Support::ensure_required_markup() will put this in the optimal order.
 			}
 			return;
 		}
@@ -1570,7 +1548,7 @@ private function parse_import_stylesheet( Import $item, CSSList $css_list, $opti
 			/**
 			 * CSS Doc.
 			 *
-			 * @var Document $css_document
+			 * @var CSSDocument $css_document
 			 */
 			$css_document = $parsed_stylesheet['css_document'];
 
@@ -1595,10 +1573,10 @@ private function parse_import_stylesheet( Import $item, CSSList $css_list, $opti
 	 * @return array {
 	 *    Parsed stylesheet.
 	 *
-	 *    @type Document $css_document       CSS Document.
-	 *    @type array    $validation_results Validation results, array containing arrays with error and sanitized keys.
-	 *    @type string   $stylesheet_url     Stylesheet URL, if available.
-	 *    @type string   $spec_name          Spec name.
+	 *    @type CSSDocument $css_document       CSS Document.
+	 *    @type array       $validation_results Validation results, array containing arrays with error and sanitized keys.
+	 *    @type string      $stylesheet_url     Stylesheet URL, if available.
+	 *    @type string      $spec_name          Spec name.
 	 * }
 	 */
 	private function parse_stylesheet( $stylesheet_string, $options ) {
@@ -2481,14 +2459,14 @@ static function( Selector $old_selector ) {
 	/**
 	 * Collect and store all CSS style attributes.
 	 *
-	 * Collects the CSS styles from within the HTML contained in this instance's DOMDocument.
+	 * Collects the CSS styles from within the HTML contained in this instance's Dom\Document.
 	 *
 	 * @see Retrieve array of styles using $this->get_styles() after calling this method.
 	 *
 	 * @since 0.4
 	 * @since 0.7 Modified to use element passed by XPath query.
 	 *
-	 * @note Uses recursion to traverse down the tree of DOMDocument nodes.
+	 * @note Uses recursion to traverse down the tree of Dom\Document nodes.
 	 *
 	 * @param DOMElement $element Node.
 	 */
@@ -2594,7 +2572,7 @@ private function finalize_styles() {
 			if ( ! $this->amp_custom_style_element ) {
 				$this->amp_custom_style_element = $this->dom->createElement( 'style' );
 				$this->amp_custom_style_element->setAttribute( 'amp-custom', '' );
-				$this->head->appendChild( $this->amp_custom_style_element );
+				$this->dom->head->appendChild( $this->amp_custom_style_element );
 			}
 
 			/*
@@ -2751,37 +2729,31 @@ private function finalize_styles() {
 			$link = $this->dom->createElement( 'link' );
 			$link->setAttribute( 'rel', 'stylesheet' );
 			$link->setAttribute( 'href', $imported_font_url );
-			$this->head->appendChild( $link );
+			$this->dom->head->appendChild( $link );
 		}
 
 		// Add style[amp-keyframes] to document.
 		if ( $stylesheet_groups[ self::STYLE_AMP_KEYFRAMES_GROUP_INDEX ]['included_count'] > 0 ) {
-			$body = $this->dom->getElementsByTagName( 'body' )->item( 0 );
-			if ( ! $body ) {
-				$body = $this->dom->createElement( 'body' );
-				$this->dom->documentElement->appendChild( $body );
-			} else {
-				$css = $stylesheet_groups[ self::STYLE_AMP_KEYFRAMES_GROUP_INDEX ]['import_front_matter'];
-
-				$css .= implode(
-					'',
-					wp_list_pluck(
-						array_filter(
-							$this->pending_stylesheets,
-							static function( $pending_stylesheet ) {
-								return $pending_stylesheet['included'] && self::STYLE_AMP_KEYFRAMES_GROUP_INDEX === $pending_stylesheet['group'];
-							}
-						),
-						'stylesheet'
-					)
-				);
-				$css .= $stylesheet_groups[ self::STYLE_AMP_KEYFRAMES_GROUP_INDEX ]['source_map_comment'];
+			$css = $stylesheet_groups[ self::STYLE_AMP_KEYFRAMES_GROUP_INDEX ]['import_front_matter'];
 
-				$style_element = $this->dom->createElement( 'style' );
-				$style_element->setAttribute( 'amp-keyframes', '' );
-				$style_element->appendChild( $this->dom->createTextNode( $css ) );
-				$body->appendChild( $style_element );
-			}
+			$css .= implode(
+				'',
+				wp_list_pluck(
+					array_filter(
+						$this->pending_stylesheets,
+						static function( $pending_stylesheet ) {
+							return $pending_stylesheet['included'] && self::STYLE_AMP_KEYFRAMES_GROUP_INDEX === $pending_stylesheet['group'];
+						}
+					),
+					'stylesheet'
+				)
+			);
+			$css .= $stylesheet_groups[ self::STYLE_AMP_KEYFRAMES_GROUP_INDEX ]['source_map_comment'];
+
+			$style_element = $this->dom->createElement( 'style' );
+			$style_element->setAttribute( 'amp-keyframes', '' );
+			$style_element->appendChild( $this->dom->createTextNode( $css ) );
+			$this->dom->body->appendChild( $style_element );
 		}
 
 		$this->remove_admin_bar_if_css_excluded();
@@ -2823,11 +2795,10 @@ private function remove_admin_bar_if_css_excluded() {
 		if ( ! $included ) {
 			// Remove admin-bar class from body element.
 			// @todo It would be nice if any style rules which refer to .admin-bar could also be removed, but this would mean retroactively going back over the CSS again and re-shaking it.
-			$body = $this->dom->getElementsByTagName( 'body' )->item( 0 );
-			if ( $body instanceof DOMElement && $body->hasAttribute( 'class' ) ) {
-				$body->setAttribute(
+			if ( $this->dom->body->hasAttribute( 'class' ) ) {
+				$this->dom->body->setAttribute(
 					'class',
-					preg_replace( '/(^|\s)admin-bar(\s|$)/', ' ', $body->getAttribute( 'class' ) )
+					preg_replace( '/(^|\s)admin-bar(\s|$)/', ' ', $this->dom->body->getAttribute( 'class' ) )
 				);
 			}
 
diff --git a/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php b/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php
index 788606abc7f..43284117bdf 100644
--- a/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php
+++ b/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Strips the tags and attributes from the content that are not allowed by the AMP spec.
  *
@@ -138,8 +140,8 @@ class AMP_Tag_And_Attribute_Sanitizer extends AMP_Base_Sanitizer {
 	 *
 	 * @since 0.5
 	 *
-	 * @param DOMDocument $dom  DOM.
-	 * @param array       $args Args.
+	 * @param Document $dom  DOM.
+	 * @param array    $args Args.
 	 */
 	public function __construct( $dom, $args = [] ) {
 		// @todo It is pointless to have this DEFAULT_ARGS copying the array values. We should only get the data from AMP_Allowed_Tags_Generated.
@@ -287,7 +289,7 @@ private function process_alternate_names( $attr_spec_list ) {
 	}
 
 	/**
-	 * Sanitize the elements from the HTML contained in this instance's DOMDocument.
+	 * Sanitize the elements from the HTML contained in this instance's Dom\Document.
 	 *
 	 * @since 0.5
 	 */
@@ -736,7 +738,7 @@ static function ( $validation_error ) {
 		// Check if element needs amp-bind component.
 		if ( $node instanceof DOMElement && ! in_array( 'amp-bind', $this->script_components, true ) ) {
 			foreach ( $node->attributes as $name => $value ) {
-				if ( AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX === substr( $name, 0, 14 ) ) {
+				if ( Document::AMP_BIND_DATA_ATTR_PREFIX === substr( $name, 0, 14 ) ) {
 					$script_components[] = 'amp-bind';
 					break;
 				}
@@ -1387,7 +1389,7 @@ private function check_attr_spec_rule_valid_url( DOMElement $node, $attr_name, $
 			foreach ( $this->extract_attribute_urls( $node->getAttributeNode( $attr_name ) ) as $url ) {
 				$url = urldecode( $url );
 
-				// Check whether the URL is parseable.
+				// Check whether the URL is parsable.
 				$parts = wp_parse_url( $url );
 				if ( false === $parts ) {
 					return AMP_Rule_Spec::FAIL;
@@ -1701,7 +1703,7 @@ private function is_amp_allowed_attribute( DOMAttr $attr_node, $attr_spec_list )
 		if (
 			isset( $attr_spec_list[ $attr_name ] )
 			||
-			( 'data-' === substr( $attr_name, 0, 5 ) && AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX !== substr( $attr_name, 0, 14 ) )
+			( 'data-' === substr( $attr_name, 0, 5 ) && Document::AMP_BIND_DATA_ATTR_PREFIX !== substr( $attr_name, 0, 14 ) )
 			||
 			// Allow the 'amp' or '⚡' attribute in <html>, like <html ⚡>.
 			( 'html' === $attr_node->parentNode->nodeName && in_array( $attr_node->nodeName, [ 'amp', '⚡' ], true ) )
diff --git a/includes/sanitizers/class-amp-video-sanitizer.php b/includes/sanitizers/class-amp-video-sanitizer.php
index 7fadfc8b201..06efaf7d9a3 100644
--- a/includes/sanitizers/class-amp-video-sanitizer.php
+++ b/includes/sanitizers/class-amp-video-sanitizer.php
@@ -45,7 +45,7 @@ public function get_selector_conversion_mapping() {
 	}
 
 	/**
-	 * Sanitize the <video> elements from the HTML contained in this instance's DOMDocument.
+	 * Sanitize the <video> elements from the HTML contained in this instance's Dom\Document.
 	 *
 	 * @since 0.2
 	 * @since 1.0 Set the filtered child node's src attribute.
diff --git a/includes/sanitizers/trait-amp-noscript-fallback.php b/includes/sanitizers/trait-amp-noscript-fallback.php
index 2276a520938..91b22227fb2 100644
--- a/includes/sanitizers/trait-amp-noscript-fallback.php
+++ b/includes/sanitizers/trait-amp-noscript-fallback.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Trait AMP_Noscript_Fallback
  *
@@ -69,11 +71,11 @@ protected function is_inside_amp_noscript( DOMNode $node ) {
 	 *
 	 * @since 1.1
 	 *
-	 * @param DOMNode     $new_node New node to append a noscript with the old node to.
-	 * @param DOMNode     $old_node Old node to append in a noscript.
-	 * @param DOMDocument $dom DOM document instance.
+	 * @param DOMNode  $new_node New node to append a noscript with the old node to.
+	 * @param DOMNode  $old_node Old node to append in a noscript.
+	 * @param Document $dom DOM document instance.
 	 */
-	protected function append_old_node_noscript( DOMNode $new_node, DOMNode $old_node, DOMDocument $dom ) {
+	protected function append_old_node_noscript( DOMNode $new_node, DOMNode $old_node, Document $dom ) {
 		$noscript = $dom->createElement( 'noscript' );
 		$noscript->appendChild( $old_node );
 		$new_node->appendChild( $noscript );
diff --git a/includes/templates/class-amp-content-sanitizer.php b/includes/templates/class-amp-content-sanitizer.php
index 6fca3c5e5a2..b4a94f7c5a5 100644
--- a/includes/templates/class-amp-content-sanitizer.php
+++ b/includes/templates/class-amp-content-sanitizer.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_Content_Sanitizer
  *
@@ -45,9 +47,9 @@ public static function sanitize( $content, array $sanitizer_classes, $global_arg
 	 *
 	 * @since 0.7
 	 *
-	 * @param DOMDocument $dom               HTML document.
-	 * @param string[]    $sanitizer_classes Sanitizer classes.
-	 * @param array       $args              Global args passed into sanitizers.
+	 * @param Document $dom               HTML document.
+	 * @param string[] $sanitizer_classes Sanitizer classes.
+	 * @param array    $args              Global args passed into sanitizers.
 	 * @return array {
 	 *     Scripts and stylesheets needed by sanitizers.
 	 *
@@ -56,7 +58,7 @@ public static function sanitize( $content, array $sanitizer_classes, $global_arg
 	 *     @type array $styles      Styles. If $args['return_styles'] is not empty. For legacy purposes.
 	 * }
 	 */
-	public static function sanitize_document( &$dom, $sanitizer_classes, $args ) {
+	public static function sanitize_document( Document $dom, $sanitizer_classes, $args ) {
 		$scripts     = [];
 		$stylesheets = [];
 		$styles      = [];
diff --git a/includes/utils/class-amp-dom-utils.php b/includes/utils/class-amp-dom-utils.php
index 68e40fdf73e..f033ea8cb30 100644
--- a/includes/utils/class-amp-dom-utils.php
+++ b/includes/utils/class-amp-dom-utils.php
@@ -5,10 +5,12 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_DOM_Utils
  *
- * Functionality to simplify working with DOMDocuments and DOMElements.
+ * Functionality to simplify working with Dom\Documents and DOMElements.
  */
 class AMP_DOM_Utils {
 
@@ -16,9 +18,10 @@ class AMP_DOM_Utils {
 	 * Attribute prefix for AMP-bind data attributes.
 	 *
 	 * @since 1.2.1
+	 * @deprecated Use Amp\AmpWP\Dom\Document::AMP_BIND_DATA_ATTR_PREFIX instead.
 	 * @var string
 	 */
-	const AMP_BIND_DATA_ATTR_PREFIX = 'data-amp-bind-';
+	const AMP_BIND_DATA_ATTR_PREFIX = Document::AMP_BIND_DATA_ATTR_PREFIX;
 
 	/**
 	 * Regular expression pattern to match events and actions within an 'on' attribute.
@@ -37,187 +40,27 @@ class AMP_DOM_Utils {
 	const AMP_ACTION_REGEX_PATTERN = '/(?<action>[^(),\s]+(?:\([^\)]+\))?)+/';
 
 	/**
-	 * HTML elements that are self-closing.
-	 *
-	 * Not all are valid AMP, but we include them for completeness.
+	 * Regular expression pattern to match the contents of the <body> element.
 	 *
-	 * @since 0.7
-	 * @link https://www.w3.org/TR/html5/syntax.html#serializing-html-fragments
-	 * @var array
-	 */
-	private static $self_closing_tags = [
-		'area',
-		'base',
-		'basefont',
-		'bgsound',
-		'br',
-		'col',
-		'embed',
-		'frame',
-		'hr',
-		'img',
-		'input',
-		'keygen',
-		'link',
-		'meta',
-		'param',
-		'source',
-		'track',
-		'wbr',
-	];
-
-	/**
-	 * List of elements allowed in head.
-	 *
-	 * @link https://github.com/ampproject/amphtml/blob/445d6e3be8a5063e2738c6f90fdcd57f2b6208be/validator/engine/htmlparser.js#L83-L100
-	 * @link https://www.w3.org/TR/html5/document-metadata.html
-	 * @var array
-	 */
-	private static $elements_allowed_in_head = [
-		'title',
-		'base',
-		'link',
-		'meta',
-		'style',
-		'noscript',
-		'script',
-	];
-
-	/**
-	 * Stored noscript/comment replacements for libxml<2.8.
-	 *
-	 * @since 0.7
-	 * @var array
+	 * @since 1.5.0
+	 * @var string
 	 */
-	public static $noscript_placeholder_comments = [];
+	const HTML_BODY_CONTENTS_PATTERN = '#^.*?<body.*?>(.*)</body>.*?$#si';
 
 	/**
-	 * Return a valid DOMDocument representing HTML document passed as a parameter.
+	 * Return a valid Dom\Document representing HTML document passed as a parameter.
 	 *
 	 * @since 0.7
 	 * @see AMP_DOM_Utils::get_content_from_dom_node()
+	 * @deprecated Use Amp\AmpWP\Dom\Document::from_html( $html, $encoding ) instead.
 	 *
-	 * @param string $document Valid HTML document to be represented by a DOMDocument.
-	 * @return DOMDocument|false Returns DOMDocument, or false if conversion failed.
+	 * @param string $document Valid HTML document to be represented by a Dom\Document.
+	 * @param string $encoding Optional. Encoding to use for the content.
+	 * @return Document|false Returns Dom\Document, or false if conversion failed.
 	 */
-	public static function get_dom( $document ) {
-		$libxml_previous_state = libxml_use_internal_errors( true );
-
-		$dom = new DOMDocument();
-
-		// @todo In the future consider an AMP_DOMDocument subclass that does this automatically. See <https://github.com/ampproject/amp-wp/pull/895/files#r163825513>.
-		$document = self::convert_amp_bind_attributes( $document );
-
-		// Force all self-closing tags to have closing tags since DOMDocument isn't fully aware.
-		$document = preg_replace(
-			'#<(' . implode( '|', self::$self_closing_tags ) . ')[^>]*>(?!</\1>)#',
-			'$0</$1>',
-			$document
-		);
-
-		// Deal with bugs in older versions of libxml.
-		$added_back_compat_meta_content_type = false;
-		if ( version_compare( LIBXML_DOTTED_VERSION, '2.8', '<' ) ) {
-			/*
-			 * Replace noscript elements with placeholders since libxml<2.8 can parse them incorrectly.
-			 * When appearing in the head element, a noscript can cause the head to close prematurely
-			 * and the noscript gets moved to the body and anything after it which was in the head.
-			 * See <https://stackoverflow.com/questions/39013102/why-does-noscript-move-into-body-tag-instead-of-head-tag>.
-			 * This is limited to only running in the head element because this is where the problem lies,
-			 * and it is important for the AMP_Script_Sanitizer to be able to access the noscript elements
-			 * in the body otherwise.
-			 */
-			$document = preg_replace_callback(
-				'#^.+?(?=<body)#is',
-				static function( $head_matches ) {
-					return preg_replace_callback(
-						'#<noscript[^>]*>.*?</noscript>#si',
-						static function( $noscript_matches ) {
-							$placeholder = sprintf( '<!--noscript:%s-->', (string) wp_rand() );
-							AMP_DOM_Utils::$noscript_placeholder_comments[ $placeholder ] = $noscript_matches[0];
-							return $placeholder;
-						},
-						$head_matches[0]
-					);
-				},
-				$document
-			);
-		}
-
-		/*
-		 * Add a pre-HTML5-style declaration of the encoding since libxml doesn't always recognize
-		 * HTML5's meta charset. In libxml<2.8 it never does, see <https://bugzilla.gnome.org/show_bug.cgi?id=655218>.
-		 * In libxml>=2.8, if the meta charset does not appear at the beginning of the head then it fails to be understood.
-		 */
-		$document = preg_replace(
-			'#(?=<meta\s+charset=["\']?([a-z0-9_-]+))#i',
-			'<meta http-equiv="Content-Type" content="text/html; charset=$1" id="meta-http-equiv-content-type">',
-			$document,
-			1,
-			$count
-		);
-		if ( 1 === $count ) {
-			$added_back_compat_meta_content_type = true;
-		}
-
-		/*
-		 * Wrap in dummy tags, since XML needs one parent node.
-		 * It also makes it easier to loop through nodes.
-		 * We can later use this to extract our nodes.
-		 * Add charset so loadHTML does not have problems parsing it.
-		 */
-		$result = $dom->loadHTML( $document );
-
-		libxml_clear_errors();
-		libxml_use_internal_errors( $libxml_previous_state );
-
-		if ( ! $result ) {
-			return false;
-		}
-
-		// Remove pre-HTML5-style encoding declaration if added above.
-		if ( $added_back_compat_meta_content_type ) {
-			$meta_http_equiv_element = $dom->getElementById( 'meta-http-equiv-content-type' );
-			if ( $meta_http_equiv_element ) {
-				$meta_http_equiv_element->parentNode->removeChild( $meta_http_equiv_element );
-			}
-		}
-
-		// Make sure there is a head and a body.
-		$head = $dom->getElementsByTagName( 'head' )->item( 0 );
-		if ( ! $head ) {
-			$head = $dom->createElement( 'head' );
-			$dom->documentElement->insertBefore( $head, $dom->documentElement->firstChild );
-		}
-		$body = $dom->getElementsByTagName( 'body' )->item( 0 );
-		if ( ! $body ) {
-			$body = $dom->createElement( 'body' );
-			$dom->documentElement->appendChild( $body );
-		}
-		self::move_invalid_head_nodes_to_body( $head, $body );
-
-		return $dom;
-	}
-
-	/**
-	 * Move elements not allowed in the head to the body.
-	 *
-	 * Apparently PHP's DOM is more lenient when parsing HTML to allow nodes in the HEAD which do not belong. A proper
-	 * HTML5 parser should rather prematurely short-circuit the HEAD when it finds an illegal element.
-	 *
-	 * @param DOMElement $head HEAD element.
-	 * @param DOMElement $body BODY element.
-	 */
-	private static function move_invalid_head_nodes_to_body( $head, $body ) {
-		// Walking backwards makes it easier to move elements in the expected order.
-		$node = $head->lastChild;
-		while ( $node ) {
-			$next_sibling = $node->previousSibling;
-			if ( ! self::is_valid_head_node( $node ) ) {
-				$body->insertBefore( $head->removeChild( $node ), $body->firstChild );
-			}
-			$node = $next_sibling;
-		}
+	public static function get_dom( $document, $encoding = null ) {
+		_deprecated_function( __METHOD__, '1.5.0', 'Amp\AmpWP\Dom\Document::from_html()' );
+		return Document::from_html( $document, $encoding );
 	}
 
 	/**
@@ -225,18 +68,14 @@ private static function move_invalid_head_nodes_to_body( $head, $body ) {
 	 *
 	 * @link https://github.com/ampproject/amphtml/blob/445d6e3be8a5063e2738c6f90fdcd57f2b6208be/validator/engine/htmlparser.js#L83-L100
 	 * @link https://www.w3.org/TR/html5/document-metadata.html
+	 * @deprecated Use Amp\AmpWP\Dom\Document->is_valid_head_node() instead.
 	 *
 	 * @param DOMNode $node Node.
 	 * @return bool Whether valid head node.
 	 */
 	public static function is_valid_head_node( DOMNode $node ) {
-		return (
-			( $node instanceof DOMElement && in_array( $node->nodeName, self::$elements_allowed_in_head, true ) )
-			||
-			( $node instanceof DOMText && preg_match( '/^\s*$/', $node->nodeValue ) ) // Whitespace text nodes are OK.
-			||
-			$node instanceof DOMComment
-		);
+		_deprecated_function( __METHOD__, '1.5.0', 'Amp\AmpWP\Dom\Document->is_valid_head_node()' );
+		return Document::from_node( $node )->is_valid_head_node( $node );
 	}
 
 	/**
@@ -248,46 +87,14 @@ public static function is_valid_head_node( DOMNode $node ) {
 	 * @since 0.7
 	 * @see \AMP_DOM_Utils::convert_amp_bind_attributes()
 	 * @see \AMP_DOM_Utils::restore_amp_bind_attributes()
-	 * @deprecated Use AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX alone.
+	 * @deprecated Use Amp\AmpWP\Dom\Document::AMP_BIND_DATA_ATTR_PREFIX instead.
 	 * @link https://www.ampproject.org/docs/reference/components/amp-bind
 	 *
 	 * @return string HTML5 data-* attribute name prefix for AMP binding attributes.
 	 */
 	public static function get_amp_bind_placeholder_prefix() {
 		_deprecated_function( __METHOD__, '1.2.1' );
-		return self::AMP_BIND_DATA_ATTR_PREFIX;
-	}
-
-	/**
-	 * Get amp-mustache tag/placeholder mappings.
-	 *
-	 * @since 0.7
-	 * @see \wpdb::placeholder_escape()
-	 *
-	 * @return array Mapping of mustache tag token to its placeholder.
-	 */
-	private static function get_mustache_tag_placeholders() {
-		static $placeholders;
-		if ( ! isset( $placeholders ) ) {
-			$salt = wp_rand();
-
-			// Note: The order of these tokens is important, as it determines the order of the order of the replacements.
-			$tokens       = [
-				'{{{',
-				'}}}',
-				'{{#',
-				'{{^',
-				'{{/',
-				'{{/',
-				'{{',
-				'}}',
-			];
-			$placeholders = [];
-			foreach ( $tokens as $token ) {
-				$placeholders[ $token ] = '_amp_mustache_' . md5( $salt . $token );
-			}
-		}
-		return $placeholders;
+		return Document::AMP_BIND_DATA_ATTR_PREFIX;
 	}
 
 	/**
@@ -299,6 +106,7 @@ private static function get_mustache_tag_placeholders() {
 	 * This is a reciprocal function of AMP_DOM_Utils::restore_amp_bind_attributes().
 	 *
 	 * @since 0.7
+	 * @deprecated This is handled automatically via Amp\AmpWP\Dom\Document.
 	 * @see \AMP_DOM_Utils::convert_amp_bind_attributes()
 	 * @link https://www.ampproject.org/docs/reference/components/amp-bind
 	 *
@@ -306,75 +114,8 @@ private static function get_mustache_tag_placeholders() {
 	 * @return string HTML with AMP binding attributes replaced with HTML5 data-* attributes.
 	 */
 	public static function convert_amp_bind_attributes( $html ) {
-
-		// Pattern for HTML attribute accounting for binding attr name, boolean attribute, single/double-quoted attribute value, and unquoted attribute values.
-		$attr_regex = '#^\s+(?P<name>\[?[a-zA-Z0-9_\-]+\]?)(?P<value>=(?:"[^"]*+"|\'[^\']*+\'|[^\'"\s]+))?#';
-
-		/**
-		 * Replace callback.
-		 *
-		 * @param array $tag_matches Tag matches.
-		 * @return string Replacement.
-		 */
-		$replace_callback = static function( $tag_matches ) use ( $attr_regex ) {
-
-			// Strip the self-closing slash as long as it is not an attribute value, like for the href attribute (<a href=/>).
-			$old_attrs = preg_replace( '#(?<!=)/$#', '', $tag_matches['attrs'] );
-
-			$old_attrs = rtrim( $old_attrs );
-
-			$new_attrs = '';
-			$offset    = 0;
-			while ( preg_match( $attr_regex, substr( $old_attrs, $offset ), $attr_matches ) ) {
-				$offset += strlen( $attr_matches[0] );
-
-				if ( '[' === $attr_matches['name'][0] ) {
-					$new_attrs .= ' ' . self::AMP_BIND_DATA_ATTR_PREFIX . trim( $attr_matches['name'], '[]' );
-					if ( isset( $attr_matches['value'] ) ) {
-						$new_attrs .= $attr_matches['value'];
-					}
-				} else {
-					$new_attrs .= $attr_matches[0];
-				}
-			}
-
-			// Bail on parse error which occurs when the regex isn't able to consume the entire $new_attrs string.
-			if ( strlen( $old_attrs ) !== $offset ) {
-				return $tag_matches[0];
-			}
-
-			return '<' . $tag_matches['name'] . $new_attrs . '>';
-		};
-
-		// Match all start tags that contain a binding attribute.
-		$pattern   = implode(
-			'',
-			[
-				'#<',
-				'(?P<name>[a-zA-Z0-9_\-]+)',               // Tag name.
-				'(?P<attrs>\s',                            // Attributes.
-				'(?:[^>"\'\[\]]+|"[^"]*+"|\'[^\']*+\')*+', // Non-binding attributes tokens.
-				'\[[a-zA-Z0-9_\-]+\]',                     // One binding attribute key.
-				'(?:[^>"\']+|"[^"]*+"|\'[^\']*+\')*+',     // Any attribute tokens, including binding ones.
-				')>#s',
-			]
-		);
-		$converted = preg_replace_callback(
-			$pattern,
-			$replace_callback,
-			$html
-		);
-
-		/**
-		 * If the regex engine incurred an error during processing, for example exceeding the backtrack
-		 * limit, $converted will be null. In this case we return the originally passed document to allow
-		 * DOMDocument to attempt to load it.  If the AMP HTML doesn't make use of amp-bind or similar
-		 * attributes, then everything should still work.
-		 *
-		 * See https://github.com/ampproject/amp-wp/issues/993 for additional context on this issue.
-		 * See http://php.net/manual/en/pcre.constants.php for additional info on PCRE errors.
-		 */
-		return ( null !== $converted ) ? $converted : $html;
+		_deprecated_function( __METHOD__, '1.5.0' );
+		return $html;
 	}
 
 	/**
@@ -384,7 +125,7 @@ public static function convert_amp_bind_attributes( $html ) {
 	 *
 	 * @since 0.7
 	 * @see \AMP_DOM_Utils::convert_amp_bind_attributes()
-	 * @deprecated Allow the data-amp-bind-* attributes to be used instead.
+	 * @deprecated This is handled automatically via Amp\AmpWP\Dom\Document.
 	 * @link https://www.ampproject.org/docs/reference/components/amp-bind
 	 *
 	 * @param string $html HTML with amp-bind attributes converted.
@@ -392,64 +133,51 @@ public static function convert_amp_bind_attributes( $html ) {
 	 */
 	public static function restore_amp_bind_attributes( $html ) {
 		_deprecated_function( __METHOD__, '1.2.1' );
-		$html = preg_replace(
-			'#\s' . self::AMP_BIND_DATA_ATTR_PREFIX . '([a-zA-Z0-9_\-]+)#',
-			' [$1]',
-			$html
-		);
 		return $html;
 	}
 
 	/**
-	 * Return a valid DOMDocument representing arbitrary HTML content passed as a parameter.
+	 * Return a valid Dom\Document representing arbitrary HTML content passed as a parameter.
 	 *
 	 * @see Reciprocal function get_content_from_dom()
 	 *
 	 * @since 0.2
 	 *
-	 * @param string $content Valid HTML content to be represented by a DOMDocument.
+	 * @param string $content  Valid HTML content to be represented by a Dom\Document.
+	 * @param string $encoding Optional. Encoding to use for the content. Defaults to `get_bloginfo( 'charset' )`.
 	 *
-	 * @return DOMDocument|false Returns DOMDocument, or false if conversion failed.
+	 * @return Document|false Returns a DOM document, or false if conversion failed.
 	 */
-	public static function get_dom_from_content( $content ) {
+	public static function get_dom_from_content( $content, $encoding = null ) {
+		// Detect encoding from the current WordPress installation.
+		if ( null === $encoding ) {
+			$encoding = get_bloginfo( 'charset' );
+		}
+
 		/*
 		 * Wrap in dummy tags, since XML needs one parent node.
 		 * It also makes it easier to loop through nodes.
 		 * We can later use this to extract our nodes.
-		 * Add utf-8 charset so loadHTML does not have problems parsing it.
-		 * See: http://php.net/manual/en/domdocument.loadhtml.php#78243
 		 */
-		$document = sprintf(
-			'<html><head><meta http-equiv="content-type" content="text/html; charset=%s"></head><body>%s</body></html>',
-			get_bloginfo( 'charset' ),
-			$content
-		);
-
-		return self::get_dom( $document );
+		$document = "<html><head></head><body>{$content}</body></html>";
 
+		return Document::from_html( $document, $encoding );
 	}
 
 	/**
-	 * Return valid HTML *body* content extracted from the DOMDocument passed as a parameter.
+	 * Return valid HTML *body* content extracted from the Dom\Document passed as a parameter.
 	 *
 	 * @since 0.2
 	 * @see AMP_DOM_Utils::get_content_from_dom_node() Reciprocal function.
 	 *
-	 * @param DOMDocument $dom Represents an HTML document from which to extract HTML content.
-	 * @return string Returns the HTML content of the body element represented in the DOMDocument.
+	 * @param Document $dom Represents an HTML document from which to extract HTML content.
+	 * @return string Returns the HTML content of the body element represented in the Dom\Document.
 	 */
-	public static function get_content_from_dom( $dom ) {
-		$body = $dom->getElementsByTagName( 'body' )->item( 0 );
-
-		// The DOMDocument may contain no body. In which case return nothing.
-		if ( null === $body ) {
-			return '';
-		}
-
+	public static function get_content_from_dom( Document $dom ) {
 		return preg_replace(
-			'#^.*?<body.*?>(.*)</body>.*?$#si',
+			static::HTML_BODY_CONTENTS_PATTERN,
 			'$1',
-			self::get_content_from_dom_node( $dom, $body )
+			$dom->saveHTML( $dom->body )
 		);
 	}
 
@@ -460,152 +188,29 @@ public static function get_content_from_dom( $dom ) {
 	 * @since 0.6
 	 * @see AMP_DOM_Utils::get_dom() Where the operations in this method are mirrored.
 	 * @see AMP_DOM_Utils::get_content_from_dom() Reciprocal function.
-	 * @todo In the future consider an AMP_DOMDocument subclass that does this automatically at saveHTML(). See <https://github.com/ampproject/amp-wp/pull/895/files#r163825513>.
+	 * @deprecated Use Dom\Document->saveHtml( $node ) instead.
 	 *
-	 * @param DOMDocument $dom  Represents an HTML document.
-	 * @param DOMElement  $node Represents an HTML element of the $dom from which to extract HTML content.
+	 * @param Document   $dom  Represents an HTML document.
+	 * @param DOMElement $node Represents an HTML element of the $dom from which to extract HTML content.
 	 * @return string Returns the HTML content represented in the DOMNode
 	 */
-	public static function get_content_from_dom_node( $dom, $node ) {
-		/**
-		 * Self closing tags regex.
-		 *
-		 * @var string Regular expression to match self-closing tags
-		 *      that saveXML() has generated a closing tag for.
-		 */
-		static $self_closing_tags_regex;
-
-		/*
-		 * Cache this regex so we don't have to recreate it every call.
-		 */
-		if ( ! isset( $self_closing_tags_regex ) ) {
-			$self_closing_tags       = implode( '|', self::$self_closing_tags );
-			$self_closing_tags_regex = "#</({$self_closing_tags})>#i";
-		}
-
-		/*
-		 * Prevent amp-mustache syntax from getting URL-encoded in attributes when saveHTML is done.
-		 * While this is applying to the entire document, it only really matters inside of <template>
-		 * elements, since URL-encoding of curly braces in href attributes would not normally matter.
-		 * But when this is done inside of a <template> then it breaks Mustache. Since Mustache
-		 * is logic-less and curly braces are not unsafe for HTML, we can do a global replacement.
-		 * The replacement is done on the entire HTML document instead of just inside of the <template>
-		 * elements since it is faster and wouldn't change the outcome.
-		 */
-		$mustache_tag_placeholders = self::get_mustache_tag_placeholders();
-		$mustache_tags_replaced    = false;
-		$xpath                     = new DOMXPath( $dom );
-		$templates                 = $dom->getElementsByTagName( 'template' );
-		foreach ( $templates as $template ) {
-
-			// These attributes are the only ones that saveHTML() will URL-encode.
-			foreach ( $xpath->query( './/*/@src|.//*/@href|.//*/@action', $template ) as $attribute ) {
-				$attribute->nodeValue = str_replace(
-					array_keys( $mustache_tag_placeholders ),
-					array_values( $mustache_tag_placeholders ),
-					$attribute->nodeValue,
-					$count
-				);
-				if ( $count ) {
-					$mustache_tags_replaced = true;
-				}
-			}
-		}
-
-		if ( version_compare( PHP_VERSION, '7.3', '>=' ) ) {
-			$html = $dom->saveHTML( $node );
-		} else {
-			/*
-			 * Temporarily add fragment boundary comments in order to locate the desired node to extract from
-			 * the given HTML document. This is required because libxml seems to only preserve whitespace when
-			 * serializing when calling DOMDocument::saveHTML() on the entire document. If you pass the element
-			 * to DOMDocument::saveHTML() then formatting whitespace gets added unexpectedly. This is seen to
-			 * be fixed in PHP 7.3, but for older versions of PHP the following workaround is needed.
-			 */
-
-			/*
-			 * First make sure meta[charset] gets http-equiv and content attributes to work around issue
-			 * with $dom->saveHTML() erroneously encoding UTF-8 as HTML entities.
-			 */
-			$meta_charset = $xpath->query( '/html/head/meta[ @charset ]' )->item( 0 );
-			if ( $meta_charset ) {
-				$meta_charset->setAttribute( 'http-equiv', 'Content-Type' );
-				$meta_charset->setAttribute( 'content', sprintf( 'text/html; charset=%s', $meta_charset->getAttribute( 'charset' ) ) );
-			}
-
-			$boundary       = 'fragment_boundary:' . wp_rand();
-			$start_boundary = $boundary . ':start';
-			$end_boundary   = $boundary . ':end';
-			$comment_start  = $dom->createComment( $start_boundary );
-			$comment_end    = $dom->createComment( $end_boundary );
-			$node->parentNode->insertBefore( $comment_start, $node );
-			$node->parentNode->insertBefore( $comment_end, $node->nextSibling );
-			$html = preg_replace(
-				'/^.*?' . preg_quote( "<!--$start_boundary-->", '/' ) . '(.*)' . preg_quote( "<!--$end_boundary-->", '/' ) . '.*?\s*$/s',
-				'$1',
-				$dom->saveHTML()
-			);
-
-			// Remove meta[http-equiv] and meta[content] attributes which were added to meta[charset] for HTML serialization.
-			if ( $meta_charset ) {
-				if ( $dom->documentElement === $node ) {
-					$html = preg_replace( '#(<meta\scharset=\S+)[^<]*?>#i', '$1>', $html );
-				}
-
-				$meta_charset->removeAttribute( 'http-equiv' );
-				$meta_charset->removeAttribute( 'content' );
-			}
-
-			$node->parentNode->removeChild( $comment_start );
-			$node->parentNode->removeChild( $comment_end );
-		}
-
-		// Whitespace just causes unit tests to fail... so whitespace begone.
-		if ( '' === trim( $html ) ) {
-			return '';
-		}
-
-		// Restore amp-mustache placeholders which were replaced to prevent URL-encoded corruption by saveHTML.
-		if ( $mustache_tags_replaced ) {
-			$html = str_replace(
-				array_values( $mustache_tag_placeholders ),
-				array_keys( $mustache_tag_placeholders ),
-				$html
-			);
-		}
-
-		// Restore noscript elements which were temporarily removed to prevent libxml<2.8 parsing problems.
-		if ( version_compare( LIBXML_DOTTED_VERSION, '2.8', '<' ) ) {
-			$html = str_replace(
-				array_keys( self::$noscript_placeholder_comments ),
-				array_values( self::$noscript_placeholder_comments ),
-				$html
-			);
-		}
-
-		/*
-		 * Travis w/PHP 7.1 generates <br></br> and <hr></hr> vs. <br/> and <hr/>, respectively.
-		 * Travis w/PHP 7.x generates <source ...></source> vs. <source ... />.  Etc.
-		 * Seems like LIBXML_NOEMPTYTAG was passed, but as you can see it was not.
-		 * This does not happen in my (@mikeschinkel) local testing, btw.
-		 */
-		$html = preg_replace( $self_closing_tags_regex, '', $html );
-
-		return $html;
+	public static function get_content_from_dom_node( Document $dom, $node ) {
+		_deprecated_function( __METHOD__, '1.5.0', 'Amp\AmpWP\Dom\Document::saveHtml()' );
+		return $dom->saveHTML( $node );
 	}
 
 	/**
-	 * Create a new node w/attributes (a DOMElement) and add to the passed DOMDocument.
+	 * Create a new node w/attributes (a DOMElement) and add to the passed Dom\Document.
 	 *
 	 * @since 0.2
 	 *
-	 * @param DOMDocument $dom        A representation of an HTML document to add the new node to.
-	 * @param string      $tag        A valid HTML element tag for the element to be added.
-	 * @param string[]    $attributes One of more valid attributes for the new node.
+	 * @param Document $dom        A representation of an HTML document to add the new node to.
+	 * @param string   $tag        A valid HTML element tag for the element to be added.
+	 * @param string[] $attributes One of more valid attributes for the new node.
 	 *
 	 * @return DOMElement|false The DOMElement for the given $tag, or false on failure
 	 */
-	public static function create_node( $dom, $tag, $attributes ) {
+	public static function create_node( Document $dom, $tag, $attributes ) {
 		$node = $dom->createElement( $tag );
 		self::add_attributes_to_node( $node, $attributes );
 
@@ -673,20 +278,20 @@ public static function is_node_empty( $node ) {
 	}
 
 	/**
-	 * Forces HTML element closing tags given a DOMDocument and optional DOMElement
+	 * Forces HTML element closing tags given a Dom\Document and optional DOMElement
 	 *
 	 * @since 0.2
 	 * @deprecated
 	 *
-	 * @param DOMDocument $dom  Represents HTML document on which to force closing tags.
-	 * @param DOMElement  $node Represents HTML element to start closing tags on.
-	 *                          If not passed, defaults to first child of body.
+	 * @param Document   $dom  Represents HTML document on which to force closing tags.
+	 * @param DOMElement $node Represents HTML element to start closing tags on.
+	 *                         If not passed, defaults to first child of body.
 	 */
 	public static function recursive_force_closing_tags( $dom, $node = null ) {
 		_deprecated_function( __METHOD__, '0.7' );
 
 		if ( null === $node ) {
-			$node = $dom->getElementsByTagName( 'body' )->item( 0 );
+			$node = $dom->body;
 		}
 
 		if ( XML_ELEMENT_NODE !== $node->nodeType ) {
diff --git a/includes/validation/class-amp-validation-manager.php b/includes/validation/class-amp-validation-manager.php
index 86740dd4522..4986b6fb5ef 100644
--- a/includes/validation/class-amp-validation-manager.php
+++ b/includes/validation/class-amp-validation-manager.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_Validation_Manager
  *
@@ -27,7 +29,7 @@ class AMP_Validation_Manager {
 	const VALIDATION_ERRORS_QUERY_VAR = 'amp_validation_errors';
 
 	/**
-	 * Action name for previewing the status change for invaliud markup.
+	 * Action name for previewing the status change for invalid markup.
 	 *
 	 * @var string
 	 */
@@ -539,7 +541,7 @@ function() {
 	/**
 	 * Override validation error statuses (when requested).
 	 *
-	 * When a query var is present along with the required nonce, override the status of the status of the invalud markup
+	 * When a query var is present along with the required nonce, override the status of the status of the invalid markup
 	 * as requested.
 	 *
 	 * @since 1.5.0
@@ -1048,8 +1050,8 @@ public static function parse_source_comment( DOMComment $comment ) {
 	 * }
 	 */
 	public static function locate_sources( DOMNode $node ) {
-		$xpath    = new DOMXPath( $node->ownerDocument );
-		$comments = $xpath->query( 'preceding::comment()[ starts-with( ., "amp-source-stack" ) or starts-with( ., "/amp-source-stack" ) ]', $node );
+		$dom      = Document::from_node( $node );
+		$comments = $dom->xpath->query( 'preceding::comment()[ starts-with( ., "amp-source-stack" ) or starts-with( ., "/amp-source-stack" ) ]', $node );
 		$sources  = [];
 		$matches  = [];
 
@@ -1282,7 +1284,7 @@ public static function wrap_hook_callbacks( $hook ) {
 				/**
 				 * Reflection.
 				 *
-				 * @var ReflectionFunctionAbstract $reflection
+				 * @var ReflectionFunction|ReflectionMethod $reflection
 				 */
 				$reflection = $source['reflection'];
 				unset( $source['reflection'] ); // Omit from stored source.
@@ -1678,9 +1680,9 @@ public static function get_validate_response_data() {
 	 *
 	 * @see AMP_Validation_Manager::add_admin_bar_menu_items()
 	 *
-	 * @param DOMDocument $dom Document.
+	 * @param Document $dom Document.
 	 */
-	public static function finalize_validation( DOMDocument $dom ) {
+	public static function finalize_validation( Document $dom ) {
 		/*
 		 * Override AMP status in admin bar set in \AMP_Validation_Manager::add_admin_bar_menu_items()
 		 * when there are validation errors which have not been explicitly accepted.
diff --git a/src/Component/Carousel.php b/src/Component/Carousel.php
index ce6b2184647..3ddf48ac8ba 100644
--- a/src/Component/Carousel.php
+++ b/src/Component/Carousel.php
@@ -7,7 +7,8 @@
 
 namespace Amp\AmpWP\Component;
 
-use DOMDocument;
+use Amp\AmpWP\Dom\Document;
+use Amp\AmpWP\Dom\ElementList;
 use DOMElement;
 use AMP_DOM_Utils;
 
@@ -38,24 +39,24 @@ final class Carousel {
 	/**
 	 * An object representation of the DOM.
 	 *
-	 * @var DOMDocument
+	 * @var Document
 	 */
 	private $dom;
 
 	/**
 	 * The slides to add to the carousel, possibly images.
 	 *
-	 * @var DOMElementList
+	 * @var ElementList
 	 */
 	private $slides;
 
 	/**
 	 * Instantiates the class.
 	 *
-	 * @param DOMDocument    $dom    The dom to use to create a carousel.
-	 * @param DOMElementList $slides The slides from which to create a carousel.
+	 * @param Document    $dom    The dom to use to create a carousel.
+	 * @param ElementList $slides The slides from which to create a carousel.
 	 */
-	public function __construct( $dom, DOMElementList $slides ) {
+	public function __construct( Document $dom, ElementList $slides ) {
 		$this->dom    = $dom;
 		$this->slides = $slides;
 	}
diff --git a/src/Dom/Document.php b/src/Dom/Document.php
new file mode 100644
index 00000000000..b1d9569a933
--- /dev/null
+++ b/src/Dom/Document.php
@@ -0,0 +1,1033 @@
+<?php
+/**
+ * Class Amp\AmpWP\Dom\Document.
+ *
+ * @package AMP
+ */
+
+namespace Amp\AmpWP\Dom;
+
+use DOMComment;
+use DOMDocument;
+use DOMElement;
+use DOMNode;
+use DOMText;
+use DOMXPath;
+
+/**
+ * Class Amp\AmpWP\Dom\Document.
+ *
+ * @since 1.5
+ *
+ * @property DOMXPath   $xpath XPath query object for this document.
+ * @property DOMElement $head  The document's <head> element.
+ * @property DOMElement $body  The document's <body> element.
+ *
+ * Abstract away some of the difficulties of working with PHP's DOMDocument.
+ */
+final class Document extends DOMDocument {
+
+	/**
+	 * AMP requires the HTML markup to be encoded in UTF-8.
+	 *
+	 * @var string
+	 */
+	const AMP_ENCODING = 'utf-8';
+
+	/**
+	 * Encoding identifier to use for an unknown encoding.
+	 *
+	 * "auto" is recognized by mb_convert_encoding() as a special value.
+	 *
+	 * @var string
+	 */
+	const UNKNOWN_ENCODING = 'auto';
+
+	/**
+	 * Encoding detection order in case we have to guess.
+	 *
+	 * This list of encoding detection order is just a wild guess and might need fine-tuning over time.
+	 * If the charset was not provided explicitly, we can really only guess, as the detection can
+	 * never be 100% accurate and reliable.
+	 *
+	 * @var string
+	 */
+	const ENCODING_DETECTION_ORDER = 'UTF-8, EUC-JP, eucJP-win, JIS, ISO-2022-JP, ISO-8859-15, ISO-8859-1, ASCII';
+
+	/**
+	 * Attribute prefix for AMP-bind data attributes.
+	 *
+	 * @var string
+	 */
+	const AMP_BIND_DATA_ATTR_PREFIX = 'data-amp-bind-';
+
+	/**
+	 * Regular expression pattern to match the http-equiv meta tag.
+	 *
+	 * @var string
+	 */
+	const HTTP_EQUIV_META_TAG_PATTERN = '/<meta [^>]*?\s*http-equiv=[^>]*?>[^<]*(?:<\/meta>)?/i';
+
+	/**
+	 * Regular expression pattern to match the charset meta tag.
+	 *
+	 * @var string
+	 */
+	const CHARSET_META_TAG_PATTERN = '/<meta [^>]*?\s*charset=[^>]*?>[^<]*(?:<\/meta>)?/i';
+
+	/**
+	 * Regular expression pattern to match the main HTML structural tags.
+	 *
+	 * @var string
+	 */
+	const HTML_STRUCTURE_PATTERN = '/(?:.*?(?<doctype><!doctype(?:\s+[^>]*)?>))?(?:(?<pre_html>.*?)(?<html_start><html(?:\s+[^>]*)?>))?(?:.*?(?<head><head(?:\s+[^>]*)?>.*?<\/head\s*>))?(?:.*?(?<body><body(?:\s+[^>]*)?>.*?<\/body\s*>))?.*?(?:(?:.*(?<html_end><\/html\s*>)|.*)(?<post_html>.*))/is';
+
+	/*
+	 * Regular expressions to fetch the individual structural tags.
+	 * These patterns were optimized to avoid extreme backtracking on large documents.
+	 */
+	const HTML_STRUCTURE_DOCTYPE_PATTERN = '/^[^<]*<!doctype(?:\s+[^>]+)?>/i';
+	const HTML_STRUCTURE_HTML_START_TAG  = '/^[^<]*(?<html_start><html(?:\s+[^>]*)?>)/i';
+	const HTML_STRUCTURE_HTML_END_TAG    = '/(?:<\/html(?:\s+[^>]*)?>)[^<>]*$/i';
+	const HTML_STRUCTURE_HEAD_START_TAG  = '/^[^<]*(?:<head(?:\s+[^>]*)?>)/i';
+	const HTML_STRUCTURE_BODY_START_TAG  = '/^[^<]*(?:<body(?:\s+[^>]*)?>)/i';
+	const HTML_STRUCTURE_BODY_END_TAG    = '/(?:<\/body(?:\s+[^>]*)?>)[^<>]*$/i';
+	const HTML_STRUCTURE_HEAD_TAG        = '/^(?:[^<]*(?:<head(?:\s+[^>]*)?>).*?<\/head(?:\s+[^>]*)?>)/is';
+
+	/**
+	 * Xpath query to fetch the attributes that are being URL-encoded by saveHTML().
+	 *
+	 * @var string
+	 */
+	const XPATH_URL_ENCODED_ATTRIBUTES_QUERY = './/*/@src|.//*/@href|.//*/@action';
+
+	/**
+	 * Error message to use when the __get() is triggered for an unknown property.
+	 *
+	 * @var string
+	 */
+	const PROPERTY_GETTER_ERROR_MESSAGE = 'Undefined property: Amp\\AmpWP\\Dom\\Document::';
+
+	// Regex patterns and values used for adding and removing http-equiv charsets for compatibility.
+	const HTML_GET_HEAD_OPENING_TAG_PATTERN     = '/<head(?:\s+[^>]*)?>/i';
+	const HTML_GET_HEAD_OPENING_TAG_REPLACEMENT = '$1<meta http-equiv="content-type" content="text/html; charset=utf-8">';
+	const HTML_GET_HTTP_EQUIV_TAG_PATTERN       = '#<meta http-equiv=([\'"])content-type\1 content=([\'"])text/html; charset=utf-8\2>#i';
+	const HTML_HTTP_EQUIV_VALUE                 = 'content-type';
+	const HTML_HTTP_EQUIV_CONTENT_VALUE         = 'text/html; charset=utf-8';
+
+	// Regex patterns used for finding tags or extracting attribute values in an HTML string.
+	const HTML_FIND_TAG_WITHOUT_ATTRIBUTE_PATTERN = '/<%1$s[^>]*?>[^<]*(?:<\/%1$s>)?/i';
+	const HTML_FIND_TAG_WITH_ATTRIBUTE_PATTERN    = '/<%1$s [^>]*?\s*%2$s=[^>]*?>[^<]*(?:<\/%1$s>)?/i';
+	const HTML_EXTRACT_ATTRIBUTE_VALUE_PATTERN    = '/%s=(?:([\'"])(?<full>.*)?\1|(?<partial>[^ \'";]+))/';
+
+	// Tags constants used throughout.
+	const TAG_HEAD     = 'head';
+	const TAG_BODY     = 'body';
+	const TAG_TEMPLATE = 'template';
+
+	/**
+	 * The original encoding of how the Amp\AmpWP\Dom\Document was created.
+	 *
+	 * This is stored to do an automatic conversion to UTF-8, which is
+	 * a requirement for AMP.
+	 *
+	 * @var string
+	 */
+	private $original_encoding;
+
+	/**
+	 * Associative array of encoding mappings.
+	 *
+	 * Translates HTML charsets into encodings PHP can understand.
+	 *
+	 * @todo Turn into const array once PHP minimum is bumped to 5.6+.
+	 *
+	 * @var string[]
+	 */
+	private static $encoding_map = [
+		// Assume ISO-8859-1 for some charsets.
+		'latin-1' => 'ISO-8859-1',
+	];
+
+	/**
+	 * HTML elements that are self-closing.
+	 *
+	 * Not all are valid AMP, but we include them for completeness.
+	 *
+	 * @link https://www.w3.org/TR/html5/syntax.html#serializing-html-fragments
+	 *
+	 * @todo Turn into const array once PHP minimum is bumped to 5.6+.
+	 *
+	 * @var string[]
+	 */
+	private static $self_closing_tags = [
+		'area',
+		'base',
+		'basefont',
+		'bgsound',
+		'br',
+		'col',
+		'embed',
+		'frame',
+		'hr',
+		'img',
+		'input',
+		'keygen',
+		'link',
+		'meta',
+		'param',
+		'source',
+		'track',
+		'wbr',
+	];
+
+	/**
+	 * List of elements allowed in head.
+	 *
+	 * @link https://github.com/ampproject/amphtml/blob/445d6e3be8a5063e2738c6f90fdcd57f2b6208be/validator/engine/htmlparser.js#L83-L100
+	 * @link https://www.w3.org/TR/html5/document-metadata.html
+	 *
+	 * @todo Turn into const array once PHP minimum is bumped to 5.6+.
+	 *
+	 * @var string[]
+	 */
+	private static $elements_allowed_in_head = [
+		'title',
+		'base',
+		'link',
+		'meta',
+		'style',
+		'noscript',
+		'script',
+	];
+
+	/**
+	 * Store the placeholder comments that were generated to replace <noscript> elements.
+	 *
+	 * @see maybe_replace_noscript_elements()
+	 *
+	 * @var string[]
+	 */
+	private $noscript_placeholder_comments = [];
+
+	/**
+	 * Store whether mustache template tags were replaced and need to be restored.
+	 *
+	 * @see replace_mustache_template_tokens()
+	 *
+	 * @var bool
+	 */
+	private $mustache_tags_replaced = false;
+
+	/**
+	 * Creates a new Amp\AmpWP\Dom\Document object
+	 *
+	 * @link  https://php.net/manual/domdocument.construct.php
+	 *
+	 * @param string $version  Optional. The version number of the document as part of the XML declaration.
+	 * @param string $encoding Optional. The encoding of the document as part of the XML declaration.
+	 */
+	public function __construct( $version = '', $encoding = null ) {
+		$this->original_encoding = (string) $encoding ?: self::UNKNOWN_ENCODING;
+		parent::__construct( $version ?: '1.0', self::AMP_ENCODING );
+	}
+
+	/**
+	 * Named constructor to provide convenient way of transforming HTML into DOM.
+	 *
+	 * @param string $html     HTML to turn into a DOM.
+	 * @param string $encoding Optional. Encoding of the provided HTML string.
+	 * @return Document|false DOM generated from provided HTML, or false if the transformation failed.
+	 */
+	public static function from_html( $html, $encoding = null ) {
+		$dom = new self( '', $encoding );
+
+		if ( ! $dom->loadHTML( $html ) ) {
+			return false;
+		}
+
+		return $dom;
+	}
+
+	/**
+	 * Named constructor to provide convenient way of retrieving the DOM from a node.
+	 *
+	 * @param DOMNode $node Node to retrieve the DOM from. This is being modified by reference (!).
+	 * @return Document DOM generated from provided HTML, or false if the transformation failed.
+	 */
+	public static function from_node( DOMNode &$node ) {
+		$root = $node->ownerDocument;
+
+		// If the node is the document itself, ownerDocument returns null.
+		if ( null === $root ) {
+			$root = $node;
+		}
+
+		if ( $root instanceof Document ) {
+			return $root;
+		}
+
+		$dom = new self();
+
+		// We replace the $node by reference, to make sure the next lines of code will
+		// work as expected with the new document.
+		// Otherwise $dom and $node would refer to two different DOMDocuments.
+		$node = $dom->importNode( $root->documentElement ?: $root, true );
+		$dom->appendChild( $node );
+
+		return $dom;
+	}
+
+	/**
+	 * Reset the internal optimizations of the Document object.
+	 *
+	 * This might be needed if you are doing an operation that causes the cached
+	 * nodes and XPath objects to point to the wrong document.
+	 *
+	 * @return self Reset version of the Document object.
+	 */
+	private function reset() {
+		// Drop references to old DOM document.
+		unset( $this->xpath, $this->head, $this->body );
+
+		// Reference of the document itself doesn't change here, but might need to change in the future.
+		return $this;
+	}
+
+	/**
+	 * Load HTML from a string.
+	 *
+	 * @link  https://php.net/manual/domdocument.loadhtml.php
+	 *
+	 * @param string     $source  The HTML string.
+	 * @param int|string $options Optional. Specify additional Libxml parameters.
+	 * @return bool true on success or false on failure.
+	 */
+	public function loadHTML( $source, $options = 0 ) {
+		$this->reset();
+
+		$source = $this->convert_amp_bind_attributes( $source );
+		$source = $this->replace_self_closing_tags( $source );
+		$source = $this->normalize_document_structure( $source );
+		$source = $this->maybe_replace_noscript_elements( $source );
+
+		list( $source, $this->original_encoding ) = $this->detect_and_strip_encoding( $source );
+
+		if ( self::AMP_ENCODING !== strtolower( $this->original_encoding ) ) {
+			$source = $this->adapt_encoding( $source );
+		}
+
+		// Force-add http-equiv charset to make DOMDocument behave as it should.
+		// See: http://php.net/manual/en/domdocument.loadhtml.php#78243.
+		$source = preg_replace( self::HTML_GET_HEAD_OPENING_TAG_PATTERN, self::HTML_GET_HEAD_OPENING_TAG_REPLACEMENT, $source, 1 );
+
+		$libxml_previous_state = libxml_use_internal_errors( true );
+
+		$success = parent::loadHTML( $source, $options );
+
+		libxml_clear_errors();
+		libxml_use_internal_errors( $libxml_previous_state );
+
+		if ( $success ) {
+			// Remove http-equiv charset again.
+			$meta = $this->head->firstChild;
+			if (
+				'meta' === $meta->tagName
+				&& self::HTML_HTTP_EQUIV_VALUE === $meta->getAttribute( 'http-equiv' )
+				&& ( self::HTML_HTTP_EQUIV_CONTENT_VALUE ) === $meta->getAttribute( 'content' )
+			) {
+				$this->head->removeChild( $meta );
+			}
+		}
+
+		// Add the required utf-8 meta charset tag.
+		$charset = $this->createElement( 'meta' );
+		$charset->setAttribute( 'charset', self::AMP_ENCODING );
+		$this->head->insertBefore( $charset, $this->head->firstChild );
+
+		// Do some further clean-up.
+		$this->move_invalid_head_nodes_to_body();
+
+		return $success;
+	}
+
+	/**
+	 * Dumps the internal document into a string using HTML formatting.
+	 *
+	 * @link  https://php.net/manual/domdocument.savehtml.php
+	 *
+	 * @param DOMNode $node Optional. Parameter to output a subset of the document.
+	 * @return string The HTML, or false if an error occurred.
+	 */
+	public function saveHTML( DOMNode $node = null ) {
+		$this->replace_mustache_template_tokens();
+
+		// Force-add http-equiv charset to make DOMDocument behave as it should.
+		// See: http://php.net/manual/en/domdocument.loadhtml.php#78243.
+		$charset = $this->createElement( 'meta' );
+		$charset->setAttribute( 'http-equiv', self::HTML_HTTP_EQUIV_VALUE );
+		$charset->setAttribute( 'content', self::HTML_HTTP_EQUIV_CONTENT_VALUE );
+		$this->head->insertBefore( $charset, $this->head->firstChild );
+
+		if ( null === $node || PHP_VERSION_ID >= 70300 ) {
+			$html = parent::saveHTML( $node );
+		} else {
+			$html = $this->extract_node_via_fragment_boundaries( $node );
+		}
+
+		// Remove http-equiv charset again.
+		$html = preg_replace( self::HTML_GET_HTTP_EQUIV_TAG_PATTERN, '', $html, 1 );
+
+		$html = $this->restore_mustache_template_tokens( $html );
+		$html = $this->maybe_restore_noscript_elements( $html );
+		$html = $this->restore_self_closing_tags( $html );
+
+		// Whitespace just causes unit tests to fail... so whitespace begone.
+		if ( '' === trim( $html ) ) {
+			return '';
+		}
+
+		return $html;
+	}
+
+	/**
+	 * Extract a node's HTML via fragment boundaries.
+	 *
+	 * Temporarily adds fragment boundary comments in order to locate the desired node to extract from
+	 * the given HTML document. This is required because libxml seems to only preserve whitespace when
+	 * serializing when calling DOMDocument::saveHTML() on the entire document. If you pass the element
+	 * to DOMDocument::saveHTML() then formatting whitespace gets added unexpectedly. This is seen to
+	 * be fixed in PHP 7.3, but for older versions of PHP the following workaround is needed.
+	 *
+	 * @param DOMNode $node Node to extract the HTML for.
+	 * @return string Extracted HTML string.
+	 */
+	private function extract_node_via_fragment_boundaries( DOMNode $node ) {
+		$boundary       = 'fragment_boundary:' . wp_rand();
+		$start_boundary = $boundary . ':start';
+		$end_boundary   = $boundary . ':end';
+		$comment_start  = $this->createComment( $start_boundary );
+		$comment_end    = $this->createComment( $end_boundary );
+
+		$node->parentNode->insertBefore( $comment_start, $node );
+		$node->parentNode->insertBefore( $comment_end, $node->nextSibling );
+
+		$html = preg_replace(
+			'/^.*?' . preg_quote( "<!--{$start_boundary}-->", '/' ) . '(.*)' . preg_quote( "<!--{$end_boundary}-->", '/' ) . '.*?\s*$/s',
+			'$1',
+			parent::saveHTML()
+		);
+
+		$node->parentNode->removeChild( $comment_start );
+		$node->parentNode->removeChild( $comment_end );
+
+		return $html;
+	}
+
+	/**
+	 * Normalize the document structure.
+	 *
+	 * This makes sure the document adheres to the general structure that AMP requires:
+	 *   ```
+	 *   <!doctype html>
+	 *   <html>
+	 *     <head>
+	 *       <meta charset="utf-8">
+	 *     </head>
+	 *     <body>
+	 *     </body>
+	 *   </html>
+	 *   ```
+	 *
+	 * @param string $content Content to normalize the structure of.
+	 * @return string Normalized content.
+	 */
+	private function normalize_document_structure( $content ) {
+		$matches    = [];
+		$html_start = '<html>';
+
+		// Strip <!doctype> for now.
+		$content = preg_replace( self::HTML_STRUCTURE_DOCTYPE_PATTERN, '', $content, 1 );
+
+		// Detect and strip <html> tags.
+		if ( preg_match( self::HTML_STRUCTURE_HTML_START_TAG, $content, $matches ) ) {
+			$html_start = $matches['html_start'];
+			$content    = preg_replace( self::HTML_STRUCTURE_HTML_START_TAG, '', $content, 1 );
+			$content    = preg_replace( self::HTML_STRUCTURE_HTML_END_TAG, '', $content, 1 );
+		}
+
+		// Detect <head> and <body> tags and add as needed.
+		if ( ! preg_match( self::HTML_STRUCTURE_HEAD_START_TAG, $content, $matches ) ) {
+			if ( ! preg_match( self::HTML_STRUCTURE_BODY_START_TAG, $content, $matches ) ) {
+				// Both <head> and <body> missing.
+				$content = "<head></head><body>{$content}</body>";
+			} else {
+				// Only <head> missing.
+				$content = "<head></head>{$content}";
+			}
+		} else {
+			if ( ! preg_match( self::HTML_STRUCTURE_BODY_END_TAG, $content, $matches ) ) {
+				// Only <body> missing.
+				// @todo This is an expensive regex operation, look into further optimization.
+				$content  = preg_replace( self::HTML_STRUCTURE_HEAD_TAG, '$0<body>', $content, 1 );
+				$content .= '</body>';
+			}
+		}
+
+		$content = "{$html_start}{$content}</html>";
+
+		// Reinsert a standard doctype.
+		$content = "<!DOCTYPE html>{$content}";
+
+		return $content;
+	}
+
+	/**
+	 * Normalize the structure of the document if it was already provided as a DOM.
+	 */
+	public function normalize_dom_structure() {
+		$head = $this->getElementsByTagName( self::TAG_HEAD )->item( 0 );
+		if ( ! $head ) {
+			$this->head = $this->createElement( self::TAG_HEAD );
+			$this->insertBefore( $this->head, $this->firstChild );
+		}
+
+		$body = $this->getElementsByTagName( self::TAG_BODY )->item( 0 );
+		if ( ! $body ) {
+			$this->body = $this->createElement( self::TAG_BODY );
+			$this->appendChild( $this->body );
+		}
+
+		$this->move_invalid_head_nodes_to_body();
+	}
+
+	/**
+	 * Move invalid head nodes back to the body.
+	 */
+	private function move_invalid_head_nodes_to_body() {
+		// Walking backwards makes it easier to move elements in the expected order.
+		$node = $this->head->lastChild;
+		while ( $node ) {
+			$next_sibling = $node->previousSibling;
+			if ( ! $this->is_valid_head_node( $node ) ) {
+				$this->body->insertBefore( $this->head->removeChild( $node ), $this->body->firstChild );
+			}
+			$node = $next_sibling;
+		}
+	}
+
+	/**
+	 * Force all self-closing tags to have closing tags.
+	 *
+	 * This is needed because DOMDocument isn't fully aware of these.
+	 *
+	 * @see restore_self_closing_tags() Reciprocal function.
+	 *
+	 * @param string $html HTML string to adapt.
+	 * @return string Adapted HTML string.
+	 */
+	private function replace_self_closing_tags( $html ) {
+		static $regex_pattern = null;
+
+		if ( null === $regex_pattern ) {
+			$regex_pattern = '#<(' . implode( '|', self::$self_closing_tags ) . ')[^>]*>(?!</\1>)#';
+		}
+
+		return preg_replace( $regex_pattern, '$0</$1>', $html );
+	}
+
+	/**
+	 * Restore all self-closing tags again.
+	 *
+	 * @see replace_self_closing_tags Reciprocal function.
+	 *
+	 * @param string $html HTML string to adapt.
+	 * @return string Adapted HTML string.
+	 */
+	private function restore_self_closing_tags( $html ) {
+		static $regex_pattern = null;
+
+		if ( null === $regex_pattern ) {
+			$regex_pattern = '#</(' . implode( '|', self::$self_closing_tags ) . ')>#i';
+		}
+
+		return preg_replace( $regex_pattern, '', $html );
+	}
+
+	/**
+	 * Maybe replace noscript elements with placeholders.
+	 *
+	 * This is done because libxml<2.8 might parse them incorrectly.
+	 * When appearing in the head element, a noscript can cause the head to close prematurely
+	 * and the noscript gets moved to the body and anything after it which was in the head.
+	 * See <https://stackoverflow.com/questions/39013102/why-does-noscript-move-into-body-tag-instead-of-head-tag>.
+	 * This is limited to only running in the head element because this is where the problem lies,
+	 * and it is important for the AMP_Script_Sanitizer to be able to access the noscript elements
+	 * in the body otherwise.
+	 *
+	 * @see maybe_restore_noscript_elements() Reciprocal function.
+	 *
+	 * @param string $html HTML string to adapt.
+	 * @return string Adapted HTML string.
+	 */
+	private function maybe_replace_noscript_elements( $html ) {
+		if ( ! version_compare( LIBXML_DOTTED_VERSION, '2.8', '<' ) ) {
+			return $html;
+		}
+
+		return preg_replace_callback(
+			'#^.+?(?=<body)#is',
+			function ( $head_matches ) {
+				return preg_replace_callback(
+					'#<noscript[^>]*>.*?</noscript>#si',
+					function ( $noscript_matches ) {
+						$placeholder = sprintf( '<!--noscript:%s-->', (string) wp_rand() );
+
+						$this->noscript_placeholder_comments[ $placeholder ] = $noscript_matches[0];
+						return $placeholder;
+					},
+					$head_matches[0]
+				);
+			},
+			$html
+		);
+	}
+
+	/**
+	 * Maybe replace noscript elements with placeholders.
+	 *
+	 * This is done because libxml<2.8 might parse them incorrectly.
+	 * When appearing in the head element, a noscript can cause the head to close prematurely
+	 * and the noscript gets moved to the body and anything after it which was in the head.
+	 * See <https://stackoverflow.com/questions/39013102/why-does-noscript-move-into-body-tag-instead-of-head-tag>.
+	 * This is limited to only running in the head element because this is where the problem lies,
+	 * and it is important for the AMP_Script_Sanitizer to be able to access the noscript elements
+	 * in the body otherwise.
+	 *
+	 * @see maybe_replace_noscript_elements() Reciprocal function.
+	 *
+	 * @param string $html HTML string to adapt.
+	 * @return string Adapted HTML string.
+	 */
+	private function maybe_restore_noscript_elements( $html ) {
+		if ( ! version_compare( LIBXML_DOTTED_VERSION, '2.8', '<' ) ) {
+			return $html;
+		}
+
+		return str_replace(
+			array_keys( $this->noscript_placeholder_comments ),
+			$this->noscript_placeholder_comments,
+			$html
+		);
+	}
+
+	/**
+	 * Replace AMP binding attributes with something that libxml can parse (as HTML5 data-* attributes).
+	 *
+	 * This is necessary because attributes in square brackets are not understood in PHP and
+	 * get dropped with an error raised:
+	 * > Warning: DOMDocument::loadHTML(): error parsing attribute name
+	 *
+	 * @link https://www.ampproject.org/docs/reference/components/amp-bind
+	 *
+	 * @param string $html HTML containing amp-bind attributes.
+	 * @return string HTML with AMP binding attributes replaced with HTML5 data-* attributes.
+	 */
+	private function convert_amp_bind_attributes( $html ) {
+
+		// Pattern for HTML attribute accounting for binding attr name, boolean attribute, single/double-quoted attribute value, and unquoted attribute values.
+		$attr_regex = '#^\s+(?P<name>\[?[a-zA-Z0-9_\-]+\]?)(?P<value>=(?:"[^"]*+"|\'[^\']*+\'|[^\'"\s]+))?#';
+
+		/**
+		 * Replace callback.
+		 *
+		 * @param array $tag_matches Tag matches.
+		 * @return string Replacement.
+		 */
+		$replace_callback = static function( $tag_matches ) use ( $attr_regex ) {
+
+			// Strip the self-closing slash as long as it is not an attribute value, like for the href attribute (<a href=/>).
+			$old_attrs = rtrim( preg_replace( '#(?<!=)/$#', '', $tag_matches['attrs'] ) );
+
+			$new_attrs = '';
+			$offset    = 0;
+			while ( preg_match( $attr_regex, substr( $old_attrs, $offset ), $attr_matches ) ) {
+				$offset += strlen( $attr_matches[0] );
+
+				if ( '[' === $attr_matches['name'][0] ) {
+					$new_attrs .= ' ' . self::AMP_BIND_DATA_ATTR_PREFIX . trim( $attr_matches['name'], '[]' );
+					if ( isset( $attr_matches['value'] ) ) {
+						$new_attrs .= $attr_matches['value'];
+					}
+				} else {
+					$new_attrs .= $attr_matches[0];
+				}
+			}
+
+			// Bail on parse error which occurs when the regex isn't able to consume the entire $new_attrs string.
+			if ( strlen( $old_attrs ) !== $offset ) {
+				return $tag_matches[0];
+			}
+
+			return '<' . $tag_matches['name'] . $new_attrs . '>';
+		};
+
+		// Match all start tags that contain a binding attribute.
+		$pattern   = implode(
+			'',
+			[
+				'#<',
+				'(?P<name>[a-zA-Z0-9_\-]+)',               // Tag name.
+				'(?P<attrs>\s',                            // Attributes.
+				'(?:[^>"\'\[\]]+|"[^"]*+"|\'[^\']*+\')*+', // Non-binding attributes tokens.
+				'\[[a-zA-Z0-9_\-]+\]',                     // One binding attribute key.
+				'(?:[^>"\']+|"[^"]*+"|\'[^\']*+\')*+',     // Any attribute tokens, including binding ones.
+				')>#s',
+			]
+		);
+		$converted = preg_replace_callback(
+			$pattern,
+			$replace_callback,
+			$html
+		);
+
+		/*
+		 * If the regex engine incurred an error during processing, for example exceeding the backtrack
+		 * limit, $converted will be null. In this case we return the originally passed document to allow
+		 * DOMDocument to attempt to load it.  If the AMP HTML doesn't make use of amp-bind or similar
+		 * attributes, then everything should still work.
+		 *
+		 * See https://github.com/ampproject/amp-wp/issues/993 for additional context on this issue.
+		 * See http://php.net/manual/en/pcre.constants.php for additional info on PCRE errors.
+		 */
+		return ( null !== $converted ) ? $converted : $html;
+	}
+
+	/**
+	 * Adapt the encoding of the content.
+	 *
+	 * @param string $source Source content to adapt the encoding of.
+	 * @return string Adapted content.
+	 */
+	private function adapt_encoding( $source ) {
+		// No encoding was provided, so we need to guess.
+		if ( self::UNKNOWN_ENCODING === $this->original_encoding && function_exists( 'mb_detect_encoding' ) ) {
+			$this->original_encoding = mb_detect_encoding( $source, self::ENCODING_DETECTION_ORDER, true );
+		}
+
+		// Guessing the encoding seems to have failed, so we assume UTF-8 instead.
+		if ( empty( $this->original_encoding ) ) {
+			$this->original_encoding = self::AMP_ENCODING;
+		}
+
+		$this->original_encoding = $this->sanitize_encoding( $this->original_encoding );
+
+		$target = false;
+		if ( self::AMP_ENCODING !== strtolower( $this->original_encoding ) ) {
+			$target = function_exists( 'mb_convert_encoding' )
+				? mb_convert_encoding( $source, self::AMP_ENCODING, $this->original_encoding )
+				: false;
+		}
+
+		return false !== $target ? $target : $source;
+	}
+
+	/**
+	 * Detect the encoding of the document.
+	 *
+	 * @param string $content Content of which to detect the encoding.
+	 * @return array {
+	 *     Detected encoding of the document, or false if none.
+	 *
+	 *     @type string $content Potentially modified content.
+	 *     @type string|false $encoding Encoding of the content, or false if not detected.
+	 * }
+	 */
+	private function detect_and_strip_encoding( $content ) {
+		$encoding = self::UNKNOWN_ENCODING;
+
+		// Check for HTML 4 http-equiv meta tags.
+		foreach ( $this->find_tags( $content, 'meta', 'http-equiv' ) as $potential_http_equiv_tag ) {
+			$encoding = $this->extract_value( $potential_http_equiv_tag, 'charset' );
+			if ( false !== $encoding ) {
+				$http_equiv_tag = $potential_http_equiv_tag;
+			}
+		}
+
+		// Check for HTML 5 charset meta tag. This overrides the HTML 4 charset.
+		$charset_tag = $this->find_tag( $content, 'meta', 'charset' );
+		if ( $charset_tag ) {
+			$encoding = $this->extract_value( $charset_tag, 'charset' );
+		}
+
+		// Strip all charset tags.
+		if ( isset( $http_equiv_tag ) ) {
+			$content = str_replace( $http_equiv_tag, '', $content );
+		}
+
+		if ( $charset_tag ) {
+			$content = str_replace( $charset_tag, '', $content );
+		}
+
+		return [ $content, $encoding ];
+	}
+
+	/**
+	 * Find a given tag with a given attribute.
+	 *
+	 * If multiple tags match, this method will only return the first one.
+	 *
+	 * @param string $content   Content in which to find the tag.
+	 * @param string $element   Element of the tag.
+	 * @param string $attribute Attribute that the tag contains.
+	 * @return string[] The requested tags. Returns an empty array if none found.
+	 */
+	private function find_tags( $content, $element, $attribute = null ) {
+		$matches = [];
+		$pattern = empty( $attribute )
+			? sprintf(
+				self::HTML_FIND_TAG_WITHOUT_ATTRIBUTE_PATTERN,
+				preg_quote( $element, '/' )
+			)
+			: sprintf(
+				self::HTML_FIND_TAG_WITH_ATTRIBUTE_PATTERN,
+				preg_quote( $element, '/' ),
+				preg_quote( $attribute, '/' )
+			);
+
+		if ( preg_match( $pattern, $content, $matches ) ) {
+			return $matches;
+		}
+
+		return [];
+	}
+
+	/**
+	 * Find a given tag with a given attribute.
+	 *
+	 * If multiple tags match, this method will only return the first one.
+	 *
+	 * @param string $content   Content in which to find the tag.
+	 * @param string $element   Element of the tag.
+	 * @param string $attribute Attribute that the tag contains.
+	 * @return string|false The requested tag, or false if not found.
+	 */
+	private function find_tag( $content, $element, $attribute = null ) {
+		$matches = $this->find_tags( $content, $element, $attribute );
+
+		if ( empty( $matches ) ) {
+			return false;
+		}
+
+		return $matches[0];
+	}
+
+	/**
+	 * Extract an attribute value from an HTML tag.
+	 *
+	 * @param string $tag       Tag from which to extract the attribute.
+	 * @param string $attribute Attribute of which to extract the value.
+	 * @return string|false Extracted attribute value, false if not found.
+	 */
+	private function extract_value( $tag, $attribute ) {
+		$matches = [];
+		$pattern = sprintf(
+			self::HTML_EXTRACT_ATTRIBUTE_VALUE_PATTERN,
+			preg_quote( $attribute, '/' )
+		);
+
+		if ( preg_match( $pattern, $tag, $matches ) ) {
+			return empty( $matches['full'] ) ? $matches['partial'] : $matches['full'];
+		}
+
+		return false;
+	}
+
+	/**
+	 * Sanitize the encoding that was detected.
+	 *
+	 * If sanitization fails, it will return 'auto', letting the conversion
+	 * logic try to figure it out itself.
+	 *
+	 * @param string $encoding Encoding to sanitize.
+	 * @return string Sanitized encoding. Falls back to 'auto' on failure.
+	 */
+	private function sanitize_encoding( $encoding ) {
+		if ( ! function_exists( 'mb_list_encodings' ) ) {
+			return $encoding;
+		}
+
+		static $known_encodings = null;
+
+		if ( null === $known_encodings ) {
+			$known_encodings = array_map( 'strtolower', mb_list_encodings() );
+		}
+
+		$lc_encoding = strtolower( $encoding );
+
+		if ( isset( self::$encoding_map[ $lc_encoding ] ) ) {
+			$encoding = self::$encoding_map[ $lc_encoding ];
+		}
+
+		if ( ! in_array( $lc_encoding, $known_encodings, true ) ) {
+			return self::UNKNOWN_ENCODING;
+		}
+
+		return $encoding;
+	}
+
+	/**
+	 * Replace Mustache template tokens to safeguard them from turning into HTML entities.
+	 *
+	 * Prevents amp-mustache syntax from getting URL-encoded in attributes when saveHTML is done.
+	 * While this is applying to the entire document, it only really matters inside of <template>
+	 * elements, since URL-encoding of curly braces in href attributes would not normally matter.
+	 * But when this is done inside of a <template> then it breaks Mustache. Since Mustache
+	 * is logic-less and curly braces are not unsafe for HTML, we can do a global replacement.
+	 * The replacement is done on the entire HTML document instead of just inside of the <template>
+	 * elements since it is faster and wouldn't change the outcome.
+	 *
+	 * @see restore_mustache_template_tokens() Reciprocal function.
+	 */
+	private function replace_mustache_template_tokens() {
+		$templates = $this->getElementsByTagName( self::TAG_TEMPLATE );
+
+		if ( ! $templates || 0 === count( $templates ) ) {
+			return;
+		}
+
+		$mustache_tag_placeholders = $this->get_mustache_tag_placeholders();
+
+		foreach ( $templates as $template ) {
+			foreach ( $this->xpath->query( self::XPATH_URL_ENCODED_ATTRIBUTES_QUERY, $template ) as $attribute ) {
+				$attribute->nodeValue = str_replace(
+					array_keys( $mustache_tag_placeholders ),
+					$mustache_tag_placeholders,
+					$attribute->nodeValue,
+					$count
+				);
+				if ( $count ) {
+					$this->mustache_tags_replaced = true;
+				}
+			}
+		}
+	}
+
+	/**
+	 * Restore Mustache template tokens that were previously replaced.
+	 *
+	 * @see replace_mustache_template_tokens() Reciprocal function.
+	 *
+	 * @param string $html HTML string to adapt.
+	 * @return string Adapted HTML string.
+	 */
+	private function restore_mustache_template_tokens( $html ) {
+		if ( ! $this->mustache_tags_replaced ) {
+			return $html;
+		}
+
+		$mustache_tag_placeholders = $this->get_mustache_tag_placeholders();
+
+		return str_replace(
+			$mustache_tag_placeholders,
+			array_keys( $mustache_tag_placeholders ),
+			$html
+		);
+	}
+
+	/**
+	 * Get amp-mustache tag/placeholder mappings.
+	 *
+	 * @see \wpdb::placeholder_escape()
+	 *
+	 * @return string[] Mapping of mustache tag token to its placeholder.
+	 */
+	private function get_mustache_tag_placeholders() {
+		static $placeholders = null;
+
+		if ( null === $placeholders ) {
+			$placeholders = [];
+			$salt         = wp_rand();
+
+			// Note: The order of these tokens is important, as it determines the order of the order of the replacements.
+			$tokens = [
+				'{{{',
+				'}}}',
+				'{{#',
+				'{{^',
+				'{{/',
+				'{{/',
+				'{{',
+				'}}',
+			];
+
+			foreach ( $tokens as $token ) {
+				$placeholders[ $token ] = '_amp_mustache_' . md5( $salt . $token );
+			}
+		}
+
+		return $placeholders;
+	}
+
+	/**
+	 * Determine whether a node can be in the head.
+	 *
+	 * @link https://github.com/ampproject/amphtml/blob/445d6e3be8a5063e2738c6f90fdcd57f2b6208be/validator/engine/htmlparser.js#L83-L100
+	 * @link https://www.w3.org/TR/html5/document-metadata.html
+	 *
+	 * @param DOMNode $node Node.
+	 * @return bool Whether valid head node.
+	 */
+	public function is_valid_head_node( DOMNode $node ) {
+		return (
+			( $node instanceof DOMElement && in_array( $node->nodeName, self::$elements_allowed_in_head, true ) )
+			||
+			( $node instanceof DOMText && preg_match( '/^\s*$/', $node->nodeValue ) ) // Whitespace text nodes are OK.
+			||
+			$node instanceof DOMComment
+		);
+	}
+
+	/**
+	 * Magic getter to implement lazily-created, cached properties for the document.
+	 *
+	 * @param string $name Name of the property to get.
+	 * @return mixed Value of the property, or null if unknown property was requested.
+	 */
+	public function __get( $name ) {
+		switch ( $name ) {
+			case 'xpath':
+				$this->xpath = new DOMXPath( $this );
+				return $this->xpath;
+			case self::TAG_HEAD:
+				$this->head = $this->getElementsByTagName( self::TAG_HEAD )->item( 0 );
+				if ( null === $this->head ) {
+					// Document was assembled manually and bypassed normalisation.
+					$this->normalize_dom_structure();
+					$this->head = $this->getElementsByTagName( self::TAG_HEAD )->item( 0 );
+				}
+				return $this->head;
+			case self::TAG_BODY:
+				$this->body = $this->getElementsByTagName( self::TAG_BODY )->item( 0 );
+				if ( null === $this->body ) {
+					// Document was assembled manually and bypassed normalisation.
+					$this->normalize_dom_structure();
+					$this->body = $this->getElementsByTagName( self::TAG_BODY )->item( 0 );
+				}
+				return $this->body;
+		}
+
+		// Mimic regular PHP behavior for missing notices.
+		trigger_error( self::PROPERTY_GETTER_ERROR_MESSAGE . $name, E_NOTICE ); // phpcs:ignore WordPress.PHP.DevelopmentFunctions,WordPress.Security.EscapeOutput
+		return null;
+	}
+
+	/**
+	 * Make sure we properly reinitialize on clone.
+	 *
+	 * @return void
+	 */
+	public function __clone() {
+		$this->reset();
+	}
+}
diff --git a/src/Component/DOMElementList.php b/src/Dom/ElementList.php
similarity index 83%
rename from src/Component/DOMElementList.php
rename to src/Dom/ElementList.php
index 0694a6ee237..875e27dabbf 100644
--- a/src/Component/DOMElementList.php
+++ b/src/Dom/ElementList.php
@@ -1,24 +1,25 @@
 <?php
 /**
- * Class DOMElementList
+ * Class ElementList
  *
  * @package Amp\AmpWP
  */
 
-namespace Amp\AmpWP\Component;
+namespace Amp\AmpWP\Dom;
 
+use Amp\AmpWP\Component\CaptionedSlide;
 use IteratorAggregate;
 use Countable;
 use DOMElement;
 use ArrayIterator;
 
 /**
- * Class DOMElementList
+ * Class ElementList
  *
  * @internal
  * @since 1.5.0
  */
-final class DOMElementList implements IteratorAggregate, Countable {
+final class ElementList implements IteratorAggregate, Countable {
 
 	/**
 	 * The elements, possibly with captions.
@@ -32,7 +33,7 @@ final class DOMElementList implements IteratorAggregate, Countable {
 	 *
 	 * @param DOMElement $element The element to add, possibly an image.
 	 * @param string     $caption The caption to add, if any.
-	 * @return DOMElementList A clone of this list, with the new element added.
+	 * @return ElementList A clone of this list, with the new element added.
 	 */
 	public function add( DOMElement $element, $caption = '' ) {
 		$cloned_list             = clone $this;
diff --git a/tests/php/stubs.php b/tests/php/stubs.php
index 04b6dcc2cb0..9f2524c7822 100644
--- a/tests/php/stubs.php
+++ b/tests/php/stubs.php
@@ -13,7 +13,7 @@ public function sanitize() {
 		$node = $this->dom->createElement( 'em' );
 		$text = $this->dom->createTextNode( 'World' );
 		$node->appendChild( $text );
-		$this->dom->getElementsByTagName( 'body' )->item( 0 )->appendChild( $node );
+		$this->dom->body->appendChild( $node );
 	}
 
 	public function get_scripts() {
diff --git a/tests/php/test-amp-analytics-options.php b/tests/php/test-amp-analytics-options.php
index 61e19f184ad..53ca0a037d2 100644
--- a/tests/php/test-amp-analytics-options.php
+++ b/tests/php/test-amp-analytics-options.php
@@ -1,5 +1,7 @@
 <?php
 
+use Amp\AmpWP\Dom\Document;
+
 class AMP_Analytics_Options_Test extends WP_UnitTestCase {
 
 	/**
@@ -139,7 +141,7 @@ public function test_one_analytics_component_added() {
 
 		$libxml_previous_state = libxml_use_internal_errors( true );
 
-		$dom = new DOMDocument();
+		$dom = new Document();
 		$dom->loadHTML( $amp_rendered );
 
 		$components = $dom->getElementsByTagName( 'amp-analytics' );
@@ -172,7 +174,7 @@ public function test_two_analytics_components_added() {
 
 		$libxml_previous_state = libxml_use_internal_errors( true );
 
-		$dom = new DOMDocument();
+		$dom = new Document();
 		$dom->loadHTML( $amp_rendered );
 		$components = $dom->getElementsByTagName( 'amp-analytics' );
 
diff --git a/tests/php/test-amp-carousel.php b/tests/php/test-amp-carousel.php
index 48005320449..e35a399e841 100644
--- a/tests/php/test-amp-carousel.php
+++ b/tests/php/test-amp-carousel.php
@@ -6,7 +6,8 @@
  */
 
 use Amp\AmpWP\Component\Carousel;
-use Amp\AmpWP\Component\DOMElementList;
+use Amp\AmpWP\Dom\Document;
+use Amp\AmpWP\Dom\ElementList;
 use Amp\AmpWP\Tests\PrivateAccess;
 
 /**
@@ -24,7 +25,7 @@ class Test_Carousel extends \WP_UnitTestCase {
 	 * @return array[] An associative array, including the slides and captions, the DOM, and the expected markup.
 	 */
 	public function get_carousel_data() {
-		$dom     = new DOMDocument();
+		$dom     = new Document();
 		$src     = 'https://example.com/img.png';
 		$width   = '1200';
 		$height  = '800';
@@ -37,12 +38,12 @@ public function get_carousel_data() {
 
 		return [
 			'image_without_caption' => [
-				( new DOMElementList() )->add( $image, '' ),
+				( new ElementList() )->add( $image, '' ),
 				$dom,
 				'<amp-carousel width="' . $width . '" height="' . $height . '" type="slides" layout="responsive"><div class="slide"><amp-img src="' . $src . '" width="' . $width . '" height="' . $height . '" layout="fill" object-fit="cover"></amp-img></div></amp-carousel>',
 			],
 			'image_with_caption'    => [
-				( new DOMElementList() )->add( $image, $caption ),
+				( new ElementList() )->add( $image, $caption ),
 				$dom,
 				'<amp-carousel width="' . $width . '" height="' . $height . '" type="slides" layout="responsive"><div class="slide"><amp-img src="' . $src . '" width="' . $width . '" height="' . $height . '" layout="fill" object-fit="cover"></amp-img><div class="amp-wp-gallery-caption"><span>' . $caption . '</span></div></div></amp-carousel>',
 			],
@@ -55,30 +56,30 @@ public function get_carousel_data() {
 	 * @dataProvider get_carousel_data
 	 * @covers \Amp\AmpWP\Component\Carousel::get_dom_element()
 	 *
-	 * @param DOMElementList $slides   An array of arrays, with images and their captions (if any).
-	 * @param DOMDocument    $dom      The representation of the DOM.
-	 * @param string         $expected The expected return value of the tested function.
+	 * @param ElementList $slides   An array of arrays, with images and their captions (if any).
+	 * @param Document    $dom      The representation of the DOM.
+	 * @param string      $expected The expected return value of the tested function.
 	 */
 	public function test_get_dom_element( $slides, $dom, $expected ) {
 		$amp_carousel        = new Carousel( $dom, $slides );
 		$actual_amp_carousel = $amp_carousel->get_dom_element();
 
 		// Prevent an error in get_content_from_dom_node().
-		$dom->appendChild( $actual_amp_carousel );
+		$dom->body->appendChild( $actual_amp_carousel );
 
 		$this->assertEquals(
 			$expected,
-			AMP_DOM_Utils::get_content_from_dom_node( $dom, $actual_amp_carousel )
+			$dom->saveHTML( $actual_amp_carousel )
 		);
 	}
 
 	/**
 	 * Gets the testing data for test_get_dimensions.
 	 *
-	 * @return array[] An associative array, including the DOMElementList and the expected return value.
+	 * @return array[] An associative array, including the ElementList and the expected return value.
 	 */
 	public function get_data_carousel_dimensions() {
-		$dom                 = new DOMDocument();
+		$dom                 = new Document();
 		$narrow_image_width  = '300';
 		$narrow_image_height = '600';
 		$narrow_image        = AMP_DOM_Utils::create_node(
@@ -112,31 +113,31 @@ public function get_data_carousel_dimensions() {
 
 		return [
 			'empty_dom_element_list_as_argument'          => [
-				( new DOMElementList() ),
+				( new ElementList() ),
 				[ Carousel::FALLBACK_WIDTH, Carousel::FALLBACK_HEIGHT ],
 			],
 			'element_no_width_or_height'                  => [
-				( new DOMElementList() )->add( AMP_DOM_Utils::create_node( $dom, 'div', [] ) ),
+				( new ElementList() )->add( AMP_DOM_Utils::create_node( $dom, 'div', [] ) ),
 				[ Carousel::FALLBACK_WIDTH, Carousel::FALLBACK_HEIGHT ],
 			],
 			'single_small_image_passed_as_argument'       => [
-				( new DOMElementList() )->add( $narrow_image ),
+				( new ElementList() )->add( $narrow_image ),
 				[ $narrow_image_width, $narrow_image_height ],
 			],
 			'single_large_image_passed_as_argument'       => [
-				( new DOMElementList() )->add( $wide_image ),
+				( new ElementList() )->add( $wide_image ),
 				[ $wide_image_width, $wide_image_height ],
 			],
 			'image_with_0_height_should_not_affect_ratio' => [
-				( new DOMElementList() )->add( $image_with_0_height )->add( $wide_image, '' ),
+				( new ElementList() )->add( $image_with_0_height )->add( $wide_image, '' ),
 				[ $wide_image_width, $wide_image_height ],
 			],
 			'two_images'                                  => [
-				( new DOMElementList() )->add( $narrow_image )->add( $wide_image, '' ),
+				( new ElementList() )->add( $narrow_image )->add( $wide_image, '' ),
 				[ $wide_image_width, $wide_image_height ],
 			],
 			'two_images_order_changed'                    => [
-				( new DOMElementList() )->add( $wide_image )->add( $narrow_image, '' ),
+				( new ElementList() )->add( $wide_image )->add( $narrow_image, '' ),
 				[ $wide_image_width, $wide_image_height ],
 			],
 		];
@@ -148,11 +149,12 @@ public function get_data_carousel_dimensions() {
 	 * @dataProvider get_data_carousel_dimensions
 	 * @covers \Amp\AmpWP\Component\Carousel::get_dimensions()
 	 *
-	 * @param DOMElementList $slides   The slides to get the dimensions from.
-	 * @param array          $expected The expected return value of the tested function.
+	 * @param ElementList $slides   The slides to get the dimensions from.
+	 * @param array       $expected The expected return value of the tested function.
+	 * @throws ReflectionException If invoking the method reflection fails.
 	 */
 	public function test_get_dimensions( $slides, $expected ) {
-		$carousel = new Carousel( new DOMDocument(), $slides );
+		$carousel = new Carousel( new Document(), $slides );
 
 		$this->assertEquals(
 			$expected,
diff --git a/tests/php/test-amp-helper-functions.php b/tests/php/test-amp-helper-functions.php
index 0935e4d7bf7..edcac5b2e6d 100644
--- a/tests/php/test-amp-helper-functions.php
+++ b/tests/php/test-amp-helper-functions.php
@@ -739,8 +739,9 @@ static function( $classes ) {
 			}
 		);
 		$ordered_sanitizers = array_keys( amp_get_content_sanitizers() );
-		$this->assertEquals( 'Even_After_Whitelist_Sanitizer', $ordered_sanitizers[ count( $ordered_sanitizers ) - 3 ] );
-		$this->assertEquals( 'AMP_Style_Sanitizer', $ordered_sanitizers[ count( $ordered_sanitizers ) - 2 ] );
+		$this->assertEquals( 'Even_After_Whitelist_Sanitizer', $ordered_sanitizers[ count( $ordered_sanitizers ) - 4 ] );
+		$this->assertEquals( 'AMP_Style_Sanitizer', $ordered_sanitizers[ count( $ordered_sanitizers ) - 3 ] );
+		$this->assertEquals( 'AMP_Meta_Sanitizer', $ordered_sanitizers[ count( $ordered_sanitizers ) - 2 ] );
 		$this->assertEquals( 'AMP_Tag_And_Attribute_Sanitizer', $ordered_sanitizers[ count( $ordered_sanitizers ) - 1 ] );
 	}
 
diff --git a/tests/php/test-amp-script-sanitizer.php b/tests/php/test-amp-script-sanitizer.php
index 5b1ad397faf..5f16b06b72f 100644
--- a/tests/php/test-amp-script-sanitizer.php
+++ b/tests/php/test-amp-script-sanitizer.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Test AMP_Script_Sanitizer.
  *
@@ -21,22 +23,22 @@ public function get_noscript_data() {
 		return [
 			'document_write'      => [
 				'<html><head></head><body>Has script? <script>document.write("Yep!")</script><noscript>Nope!</noscript></body></html>',
-				'<html><head></head><body>Has script? <!--noscript-->Nope!<!--/noscript--></body></html>',
+				'<html><head><meta charset="utf-8"></head><body>Has script? <!--noscript-->Nope!<!--/noscript--></body></html>',
 			],
 			'nested_elements'     => [
 				'<html><head></head><body><noscript>before <em><strong>middle</strong> end</em></noscript></body></html>',
-				'<html><head></head><body><!--noscript-->before <em><strong>middle</strong> end</em><!--/noscript--></body></html>',
+				'<html><head><meta charset="utf-8"></head><body><!--noscript-->before <em><strong>middle</strong> end</em><!--/noscript--></body></html>',
 			],
 			'head_noscript_style' => [
 				'<html><head><noscript><style>body{color:red}</style></noscript></head><body></body></html>',
-				'<html><head><!--noscript--><style>body{color:red}</style><!--/noscript--></head><body></body></html>',
+				'<html><head><meta charset="utf-8"><!--noscript--><style>body{color:red}</style><!--/noscript--></head><body></body></html>',
 			],
 			'head_noscript_span'  => [
 				'<html><head><noscript><span>No script</span></noscript></head><body></body></html>',
-				'<html><head></head><body><!--noscript--><span>No script</span><!--/noscript--></body></html>',
+				'<html><head><meta charset="utf-8"></head><body><!--noscript--><span>No script</span><!--/noscript--></body></html>',
 			],
 			'test_with_dev_mode'  => [
-				'<html data-ampdevmode=""><head></head><body><noscript data-ampdevmode="">hey</noscript></body></html>',
+				'<html data-ampdevmode=""><head><meta charset="utf-8"></head><body><noscript data-ampdevmode="">hey</noscript></body></html>',
 				null,
 			],
 		];
@@ -54,13 +56,13 @@ public function test_noscript_promotion( $source, $expected = null ) {
 		if ( null === $expected ) {
 			$expected = $source;
 		}
-		$dom = AMP_DOM_Utils::get_dom( $source );
+		$dom = Document::from_html( $source );
 		$this->assertSame( 1, $dom->getElementsByTagName( 'noscript' )->length );
 		$sanitizer = new AMP_Script_Sanitizer( $dom );
 		$sanitizer->sanitize();
 		$whitelist_sanitizer = new AMP_Tag_And_Attribute_Sanitizer( $dom );
 		$whitelist_sanitizer->sanitize();
-		$content = AMP_DOM_Utils::get_content_from_dom_node( $dom, $dom->documentElement );
+		$content = $dom->saveHTML( $dom->documentElement );
 		$this->assertEquals( $expected, $content );
 	}
 
@@ -103,10 +105,10 @@ public function test_boilerplate_preservation() {
 			'use_document_element' => true,
 		];
 
-		$dom = AMP_DOM_Utils::get_dom( $html );
+		$dom = Document::from_html( $html );
 		AMP_Content_Sanitizer::sanitize_document( $dom, amp_get_content_sanitizers(), $args );
 
-		$content = AMP_DOM_Utils::get_content_from_dom_node( $dom, $dom->documentElement );
+		$content = $dom->saveHTML( $dom->documentElement );
 
 		$this->assertRegExp( '/<!-- Google Tag Manager -->\s*<!-- End Google Tag Manager -->/', $content );
 		$this->assertContains( '<noscript><style amp-boilerplate>body{-webkit-animation:none;-moz-animation:none;-ms-animation:none;animation:none}</style></noscript>', $content );
diff --git a/tests/php/test-amp-style-sanitizer.php b/tests/php/test-amp-style-sanitizer.php
index b8441e8ee62..b22274e443d 100644
--- a/tests/php/test-amp-style-sanitizer.php
+++ b/tests/php/test-amp-style-sanitizer.php
@@ -7,6 +7,7 @@
 
 // phpcs:disable WordPress.Arrays.MultipleStatementAlignment.DoubleArrowNotAligned
 
+use Amp\AmpWP\Dom\Document;
 use Amp\AmpWP\Tests\PrivateAccess;
 
 /**
@@ -573,7 +574,7 @@ static function( $preempt, $request, $url ) {
 			10,
 			3
 		);
-		$dom = AMP_DOM_Utils::get_dom( $source );
+		$dom = Document::from_html( $source );
 
 		$error_codes = [];
 		$args        = [
@@ -589,7 +590,7 @@ static function( $preempt, $request, $url ) {
 		$whitelist_sanitizer = new AMP_Tag_And_Attribute_Sanitizer( $dom, $args );
 		$whitelist_sanitizer->sanitize();
 
-		$sanitized_html     = AMP_DOM_Utils::get_content_from_dom_node( $dom, $dom->documentElement );
+		$sanitized_html     = $dom->saveHTML( $dom->documentElement );
 		$actual_stylesheets = array_values( $sanitizer->get_stylesheets() );
 		$this->assertEquals( $expected_errors, $error_codes );
 		$this->assertCount( count( $expected_stylesheets ), $actual_stylesheets );
@@ -724,7 +725,7 @@ public function get_amp_selector_data() {
 	 */
 	public function test_amp_selector_conversion( $markup, $input, $output ) {
 		$html = "<html amp><head><meta charset=utf-8><style amp-custom>$input</style></head><body>$markup</body></html>";
-		$dom  = AMP_DOM_Utils::get_dom( $html );
+		$dom  = Document::from_html( $html );
 
 		$sanitizer_classes = amp_get_content_sanitizers();
 		$sanitized         = AMP_Content_Sanitizer::sanitize_document(
@@ -870,7 +871,7 @@ static function ( $selector ) {
 		);
 
 		$html = "<html amp><head><meta charset=utf-8><style amp-custom>$style</style></head><body>$markup</body></html>";
-		$dom  = AMP_DOM_Utils::get_dom( $html );
+		$dom  = Document::from_html( $html );
 
 		$sanitizer_classes = amp_get_content_sanitizers();
 
@@ -1063,7 +1064,7 @@ public function get_amp_css_hacks_data() {
 	 */
 	public function test_browser_css_hacks( $input ) {
 		$html = "<html amp><head><meta charset=utf-8><style amp-custom>$input</style></head><body></body></html>";
-		$dom  = AMP_DOM_Utils::get_dom( $html );
+		$dom  = Document::from_html( $html );
 
 		$error_codes = [];
 		$sanitizer   = new AMP_Style_Sanitizer(
@@ -1093,7 +1094,7 @@ public function test_font_data_url_handling() {
 		$html .= '</head><body><span class="b dashicons dashicons-admin-appearance"></span></body></html>';
 
 		// Test with tree-shaking.
-		$dom         = AMP_DOM_Utils::get_dom( $html );
+		$dom         = Document::from_html( $html );
 		$error_codes = [];
 		$sanitizer   = new AMP_Style_Sanitizer(
 			$dom,
@@ -1136,7 +1137,7 @@ public function test_font_data_url_handling_without_file_sources() {
 		$html .= '<style>@font-face { font-family: "Custom"; src: url("data:application/x-font-woff;charset=utf-8;base64,d09GRgABAAA") format("woff"); }</style>';
 		$html .= '</head><body></body></html>';
 
-		$dom         = AMP_DOM_Utils::get_dom( $html );
+		$dom         = Document::from_html( $html );
 		$error_codes = [];
 		$sanitizer   = new AMP_Style_Sanitizer(
 			$dom,
@@ -1203,7 +1204,7 @@ public function test_class_amp_bind_preservation() {
 			</body>
 		</html>
 		<?php
-		$dom = AMP_DOM_Utils::get_dom( ob_get_clean() );
+		$dom = Document::from_html( ob_get_clean() );
 
 		$error_codes = [];
 		$sanitizer   = new AMP_Style_Sanitizer(
@@ -1271,7 +1272,7 @@ public function test_large_custom_css_and_rule_removal() {
 			</style>
 		';
 		$html .= '</head><body><span class="b" data-value="">...</span><span id="exists"></span></body></html>';
-		$dom   = AMP_DOM_Utils::get_dom( $html );
+		$dom   = Document::from_html( $html );
 
 		$error_codes = [];
 		$sanitizer   = new AMP_Style_Sanitizer(
@@ -1344,7 +1345,7 @@ public function test_css_manifest() {
 			$html = ob_get_clean();
 
 			$error_codes = [];
-			$dom         = AMP_DOM_Utils::get_dom( $html );
+			$dom         = Document::from_html( $html );
 			$sanitizer   = new AMP_Style_Sanitizer(
 				$dom,
 				array_merge(
@@ -1358,8 +1359,7 @@ public function test_css_manifest() {
 				)
 			);
 			$sanitizer->sanitize();
-			$xpath = new DOMXPath( $dom );
-			$style = $xpath->query( '//style[ @amp-custom ]' )->item( 0 );
+			$style = $dom->xpath->query( '//style[ @amp-custom ]' )->item( 0 );
 
 			return [ $style, $error_codes ];
 		};
@@ -1433,7 +1433,7 @@ public function test_css_manifest() {
 	 */
 	public function test_relative_background_url_handling() {
 		$html = '<html amp><head><meta charset="utf-8"><link rel="stylesheet" href="' . esc_url( admin_url( 'css/common.css' ) ) . '"></head><body><span class="spinner"></span></body></html>'; // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedStylesheet
-		$dom  = AMP_DOM_Utils::get_dom( $html );
+		$dom  = Document::from_html( $html );
 
 		$sanitizer = new AMP_Style_Sanitizer(
 			$dom,
@@ -1442,7 +1442,7 @@ public function test_relative_background_url_handling() {
 			]
 		);
 		$sanitizer->sanitize();
-		AMP_DOM_Utils::get_content_from_dom_node( $dom, $dom->documentElement );
+		$dom->saveHTML( $dom->documentElement );
 		$actual_stylesheets = array_values( $sanitizer->get_stylesheets() );
 		$this->assertCount( 1, $actual_stylesheets );
 		$stylesheet = $actual_stylesheets[0];
@@ -1519,7 +1519,7 @@ static function( $preempt, $request, $url ) use ( $href, &$request_count, $conte
 
 		$sanitize_and_get_stylesheets = static function() use ( $href ) {
 			$html = sprintf( '<html amp><head><meta charset="utf-8"><link rel="stylesheet" href="%s"></head><body></body></html>', esc_url( $href ) ); // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedStylesheet
-			$dom  = AMP_DOM_Utils::get_dom( $html );
+			$dom  = Document::from_html( $html );
 
 			$found_error_codes = [];
 
@@ -1533,7 +1533,7 @@ static function( $preempt, $request, $url ) use ( $href, &$request_count, $conte
 				]
 			);
 			$sanitizer->sanitize();
-			AMP_DOM_Utils::get_content_from_dom_node( $dom, $dom->documentElement );
+			$dom->saveHTML( $dom->documentElement );
 			return [ $found_error_codes, array_values( $sanitizer->get_stylesheets() ) ];
 		};
 
@@ -1748,7 +1748,7 @@ public function get_stylesheet_urls() {
 	 * @param string      $error_code Error code. Optional.
 	 */
 	public function test_get_validated_url_file_path( $source, $expected, $error_code = null ) {
-		$dom = AMP_DOM_Utils::get_dom( '<html></html>' );
+		$dom = Document::from_html( '<html></html>' );
 
 		$sanitizer = new AMP_Style_Sanitizer( $dom );
 		$actual    = $sanitizer->get_validated_url_file_path( $source, [ 'css' ] );
@@ -1809,7 +1809,7 @@ public function test_remove_spaces_from_url_values( $source, $expected ) {
 		$html .= $source;
 		$html .= '</style></head</html>';
 
-		$dom = AMP_DOM_Utils::get_dom( $html );
+		$dom = Document::from_html( $html );
 
 		$sanitizer = new AMP_Style_Sanitizer( $dom );
 		$sanitizer->sanitize();
@@ -1865,7 +1865,7 @@ public function test_font_urls( $url, $error_codes ) {
 		$tag = sprintf( '<link rel="stylesheet" href="%s">', esc_url( $url ) ); // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedStylesheet
 		$tag = amp_filter_font_style_loader_tag_with_crossorigin_anonymous( $tag, 'font', $url );
 
-		$dom = AMP_DOM_Utils::get_dom( sprintf( '<html><head>%s</head></html>', $tag ) );
+		$dom = Document::from_html( sprintf( '<html><head>%s</head></html>', $tag ) );
 
 		$validation_errors = [];
 
@@ -1906,7 +1906,7 @@ public function test_cors_enabled_stylesheet_url() {
 		// Test supplying crossorigin attribute.
 		$url       = 'https://fonts.googleapis.com/css?family=Tangerine';
 		$link      = amp_filter_font_style_loader_tag_with_crossorigin_anonymous( "<link rel='stylesheet' href='$url'>", 'font', $url ); // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedStylesheet
-		$document  = AMP_DOM_Utils::get_dom( "<html><head>$link</head></html>" );
+		$document  = Document::from_html( "<html><head>$link</head></html>" );
 		$sanitizer = new AMP_Style_Sanitizer( $document, [ 'use_document_element' => true ] );
 		$sanitizer->sanitize();
 		$link = $document->getElementsByTagName( 'link' )->item( 0 );
@@ -1915,7 +1915,7 @@ public function test_cors_enabled_stylesheet_url() {
 
 		// Test that existing crossorigin attribute is not overridden.
 		$link      = amp_filter_font_style_loader_tag_with_crossorigin_anonymous( "<link crossorigin='use-credentials' rel='stylesheet' href='$url'>", 'font', $url ); // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedStylesheet
-		$document  = AMP_DOM_Utils::get_dom( "<html><head>$link</head></html>" ); // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedStylesheet
+		$document  = Document::from_html( "<html><head>$link</head></html>" ); // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedStylesheet
 		$sanitizer = new AMP_Style_Sanitizer( $document, [ 'use_document_element' => true ] );
 		$sanitizer->sanitize();
 		$link = $document->getElementsByTagName( 'link' )->item( 0 );
@@ -2150,7 +2150,7 @@ static function( $preempt, $request, $url ) use ( $mock_response, $stylesheet_ur
 			3
 		);
 
-		$dom = AMP_DOM_Utils::get_dom( $markup );
+		$dom = Document::from_html( $markup );
 
 		if ( ! empty( $options['auto_reject'] ) ) {
 			add_filter( 'amp_validation_error_sanitized', '__return_false' );
@@ -2184,7 +2184,7 @@ public function test_css_import_font() {
 		$markup .= sprintf( '<style>@import "%s"; body{color:red}</style>', $stylesheet_url );
 		$markup .= '</head><body>hello</body></html>';
 
-		$dom       = AMP_DOM_Utils::get_dom( $markup );
+		$dom       = Document::from_html( $markup );
 		$sanitizer = new AMP_Style_Sanitizer(
 			$dom,
 			[
@@ -2196,8 +2196,7 @@ public function test_css_import_font() {
 
 		$this->assertCount( 1, $stylesheets );
 		$this->assertEquals( 'body{color:red}', $stylesheets[0] );
-		$xpath = new DOMXPath( $dom );
-		$link  = $xpath->query( '//link[ @rel = "stylesheet" ]' )->item( 0 );
+		$link = $dom->xpath->query( '//link[ @rel = "stylesheet" ]' )->item( 0 );
 		$this->assertInstanceOf( 'DOMElement', $link );
 		$this->assertEquals( set_url_scheme( $stylesheet_url, 'https' ), $link->getAttribute( 'href' ) );
 	}
@@ -2246,7 +2245,7 @@ public function test_style_element_cdata() {
 		$html .= '<style><!--/*--><![CDATA[/*><!--*/ body { color:blue } /*]]>*/--></style>';
 		$html .= '</head><body><p>Hello World</p></body></html>';
 
-		$dom       = AMP_DOM_Utils::get_dom( $html );
+		$dom       = Document::from_html( $html );
 		$sanitizer = new AMP_Style_Sanitizer(
 			$dom,
 			[
@@ -2256,8 +2255,7 @@ public function test_style_element_cdata() {
 
 		$sanitizer->sanitize();
 
-		$xpath = new DOMXPath( $dom );
-		$style = $xpath->query( '//style[ @amp-custom ]' )->item( 0 );
+		$style = $dom->xpath->query( '//style[ @amp-custom ]' )->item( 0 );
 		$this->assertInstanceOf( 'DOMElement', $style );
 
 		$expected = "body{color:red}body{color:green}body{color:blue}\n\n/*# sourceURL=amp-custom.css */";
@@ -2271,7 +2269,7 @@ public function test_style_element_cdata() {
 	 */
 	public function test_body_font_stylesheet_moved_to_head() {
 		$html = '<!DOCTYPE html><html amp><head><meta charset="utf-8"></head><body><link rel="stylesheet" id="the-font" href="https://fonts.googleapis.com/css?family=Merriweather%3A400%2C700" type="text/css" media="all"></body></html>'; // phpcs:ignore
-		$dom  = AMP_DOM_Utils::get_dom( $html );
+		$dom  = Document::from_html( $html );
 
 		$link = $dom->getElementById( 'the-font' );
 		$this->assertInstanceOf( 'DOMElement', $link );
@@ -2354,14 +2352,14 @@ function( $original_dom, $original_source, $amphtml_dom, $amphtml_source ) {
 					/**
 					 * Vars.
 					 *
-					 * @var DOMDocument $original_dom
-					 * @var string      $original_source
-					 * @var DOMDocument $amphtml_dom
-					 * @var string      $amphtml_source
+					 * @var Document $original_dom
+					 * @var string   $original_source
+					 * @var Document $amphtml_dom
+					 * @var string   $amphtml_source
 					 */
 					$this->assertInstanceOf( 'DOMElement', $original_dom->getElementById( 'wpadminbar' ), 'Expected admin bar element to be present originally.' );
 					$this->assertInstanceOf( 'DOMElement', $original_dom->getElementById( 'admin-bar-css' ), 'Expected admin bar CSS to be present originally.' );
-					$this->assertContains( 'admin-bar', $original_dom->getElementsByTagName( 'body' )->item( 0 )->getAttribute( 'class' ) );
+					$this->assertContains( 'admin-bar', $original_dom->body->getAttribute( 'class' ) );
 					$this->assertContains( 'earlyprintstyle', $original_source, 'Expected early print style to not be present.' );
 
 					$this->assertContains( '.is-style-outline .wp-block-button__link', $amphtml_source, 'Expected block-library/style.css' );
@@ -2369,7 +2367,7 @@ function( $original_dom, $original_source, $amphtml_dom, $amphtml_source ) {
 					$this->assertContains( 'amp-img.amp-wp-enforced-sizes', $amphtml_source, 'Expected amp-default.css' );
 					$this->assertContains( 'ab-empty-item', $amphtml_source, 'Expected admin-bar.css to still be present.' );
 					$this->assertNotContains( 'earlyprintstyle', $amphtml_source, 'Expected early print style to not be present.' );
-					$this->assertContains( 'admin-bar', $amphtml_dom->getElementsByTagName( 'body' )->item( 0 )->getAttribute( 'class' ) );
+					$this->assertContains( 'admin-bar', $amphtml_dom->body->getAttribute( 'class' ) );
 					$this->assertInstanceOf( 'DOMElement', $amphtml_dom->getElementById( 'wpadminbar' ) );
 				},
 			],
@@ -2407,8 +2405,8 @@ public function test_prioritized_stylesheets( $html_generator, $assert ) {
 		$this->go_to( home_url() );
 		$html = $html_generator();
 
-		$original_dom = AMP_DOM_Utils::get_dom( $html );
-		$amphtml_dom  = clone $original_dom;
+		$original_dom = Document::from_html( $html );
+		$amphtml_dom  = clone( $original_dom );
 
 		$error_codes = [];
 		$args        = [
@@ -2627,7 +2625,7 @@ public function test_get_stylesheet_priority( $node_data, $expected ) {
 			return 'child-theme';
 		};
 
-		$dom = new DOMDocument();
+		$dom = new Document();
 
 		if ( isset( $node_data[0] ) ) {
 			$node = AMP_DOM_Utils::create_node( $dom, $node_data[0], $node_data[1] );
diff --git a/tests/php/test-amp-twitter-embed.php b/tests/php/test-amp-twitter-embed.php
index 1e4bdc3eb53..182905db9ad 100644
--- a/tests/php/test-amp-twitter-embed.php
+++ b/tests/php/test-amp-twitter-embed.php
@@ -132,7 +132,7 @@ public function test__conversion( $source, $expected ) {
 
 		$content = AMP_DOM_Utils::get_content_from_dom( $dom );
 
-		$this->assertEquals( $content, $expected );
+		$this->assertEquals( $expected, $content );
 	}
 
 	public function get_scripts_data() {
diff --git a/tests/php/test-class-amp-base-sanitizer.php b/tests/php/test-class-amp-base-sanitizer.php
index cf333237b33..4cde64bc24f 100644
--- a/tests/php/test-class-amp-base-sanitizer.php
+++ b/tests/php/test-class-amp-base-sanitizer.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Test AMP_Base_Sanitizer_Test
  *
@@ -185,7 +187,7 @@ public function get_data() {
 	 * @covers AMP_Base_Sanitizer::set_layout()
 	 */
 	public function test_set_layout( $source_attributes, $expected_attributes, $args = [] ) {
-		$sanitizer           = new AMP_Test_Stub_Sanitizer( new DOMDocument(), $args );
+		$sanitizer           = new AMP_Test_Stub_Sanitizer( new Document(), $args );
 		$returned_attributes = $sanitizer->set_layout( $source_attributes );
 		$this->assertEquals( $expected_attributes, $returned_attributes );
 	}
@@ -261,7 +263,7 @@ public function get_sanitize_dimension_data() {
 	 * @covers AMP_Base_Sanitizer::sanitize_dimension()
 	 */
 	public function test_sanitize_dimension( $source_params, $expected_value, $args = [] ) {
-		$sanitizer                 = new AMP_Test_Stub_Sanitizer( new DOMDocument(), $args );
+		$sanitizer                 = new AMP_Test_Stub_Sanitizer( new Document(), $args );
 		list( $value, $dimension ) = $source_params;
 
 		$actual_value = $sanitizer->sanitize_dimension( $value, $dimension );
@@ -278,7 +280,7 @@ public function test_sanitize_dimension( $source_params, $expected_value, $args
 	 */
 	public function test_remove_invalid_child() {
 		$parent_tag_name = 'div';
-		$dom_document    = new DOMDocument( '1.0', 'utf-8' );
+		$dom_document    = new Document( '1.0', 'utf-8' );
 		$parent          = $dom_document->createElement( $parent_tag_name );
 		$child           = $dom_document->createElement( 'script' );
 		$child->setAttribute( 'id', 'foo' );
@@ -492,14 +494,14 @@ public function test_remove_invalid_attribute_dev_mode() {
 	 */
 	public function test_get_data_amp_attributes() {
 		$tag          = 'figure';
-		$dom_document = new DOMDocument( '1.0', 'utf-8' );
+		$dom_document = new Document( '1.0', 'utf-8' );
 		$figure       = $dom_document->createElement( $tag );
 		$amp_img      = $dom_document->createElement( 'amp-img' );
 		$figure->appendChild( $amp_img );
 		$figure->setAttribute( 'data-amp-noloading', 'true' );
 		$figure->setAttribute( 'data-amp-layout', 'fixed' );
 
-		$sanitizer = new AMP_Test_Stub_Sanitizer( new DOMDocument(), [] );
+		$sanitizer = new AMP_Test_Stub_Sanitizer( new Document(), [] );
 		$amp_args  = $sanitizer->get_data_amp_attributes( $amp_img );
 
 		$expected_args = [
@@ -523,7 +525,7 @@ public function test_filter_data_amp_attributes() {
 		$attributes = [
 			'width' => 100,
 		];
-		$sanitizer  = new AMP_Test_Stub_Sanitizer( new DOMDocument(), [] );
+		$sanitizer  = new AMP_Test_Stub_Sanitizer( new Document(), [] );
 		$attributes = $sanitizer->filter_data_amp_attributes( $attributes, $amp_data );
 
 		$expected = [
@@ -539,9 +541,9 @@ public function test_filter_data_amp_attributes() {
 	 * @covers AMP_Base_Sanitizer::filter_attachment_layout_attributes()
 	 */
 	public function test_filter_attachment_layout_attributes() {
-		$sanitizer    = new AMP_Test_Stub_Sanitizer( new DOMDocument(), [] );
+		$sanitizer    = new AMP_Test_Stub_Sanitizer( new Document(), [] );
 		$tag          = 'figure';
-		$dom_document = new DOMDocument( '1.0', 'utf-8' );
+		$dom_document = new Document( '1.0', 'utf-8' );
 		$figure       = $dom_document->createElement( $tag );
 		$amp_img      = $dom_document->createElement( 'amp-img' );
 		$layout       = 'fixed-height';
diff --git a/tests/php/test-class-amp-comments-sanitizer.php b/tests/php/test-class-amp-comments-sanitizer.php
index fd63410efa4..3ff2084e6a2 100644
--- a/tests/php/test-class-amp-comments-sanitizer.php
+++ b/tests/php/test-class-amp-comments-sanitizer.php
@@ -5,6 +5,7 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
 use Amp\AmpWP\Tests\PrivateAccess;
 
 /**
@@ -19,7 +20,7 @@ class Test_AMP_Comments_Sanitizer extends WP_UnitTestCase {
 	/**
 	 * Representation of the DOM.
 	 *
-	 * @var DOMDocument
+	 * @var Document
 	 */
 	public $dom;
 
@@ -31,7 +32,7 @@ class Test_AMP_Comments_Sanitizer extends WP_UnitTestCase {
 	public function setUp() {
 		parent::setUp();
 		$GLOBALS['post'] = self::factory()->post->create_and_get();
-		$this->dom       = new DOMDocument();
+		$this->dom       = new Document();
 	}
 
 	/**
@@ -96,6 +97,11 @@ public function test_process_comment_form() {
 			$this->assertContains( $name, $amp_state->nodeValue );
 		}
 		foreach ( $form->getElementsByTagName( 'input' ) as $input ) {
+			/**
+			 * Input.
+			 *
+			 * @var DOMElement $input
+			 */
 			$on = $input->getAttribute( 'on' );
 			$this->assertContains( 'change:AMP.setState(', $on );
 			$this->assertContains( strval( $GLOBALS['post']->ID ), $on );
@@ -121,8 +127,7 @@ public function test_add_amp_live_list_comment_attributes() {
 		$this->create_comments_list( $comment_objects );
 		$instance->sanitize();
 
-		$xpath    = new DOMXPath( $this->dom );
-		$comments = $xpath->query( '//*[ starts-with( @id, "comment-" ) ]' );
+		$comments = $this->dom->xpath->query( '//*[ starts-with( @id, "comment-" ) ]' );
 
 		foreach ( $comments as $comment ) {
 			/**
diff --git a/tests/php/test-class-amp-core-theme-sanitizer.php b/tests/php/test-class-amp-core-theme-sanitizer.php
index 73702342900..a5c93ed3fd8 100644
--- a/tests/php/test-class-amp-core-theme-sanitizer.php
+++ b/tests/php/test-class-amp-core-theme-sanitizer.php
@@ -5,6 +5,7 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
 use Amp\AmpWP\Tests\PrivateAccess;
 
 /**
@@ -38,16 +39,19 @@ public function get_xpath_from_css_selector_data() {
 	 *
 	 * @dataProvider get_xpath_from_css_selector_data
 	 * @covers AMP_Core_Theme_Sanitizer::xpath_from_css_selector()
+	 *
+	 * @param string $css_selector CSS Selector.
+	 * @param string $expected     Expected XPath expression.
 	 */
 	public function test_xpath_from_css_selector( $css_selector, $expected ) {
-		$dom       = new DOMDocument();
+		$dom       = new Document();
 		$sanitizer = new AMP_Core_Theme_Sanitizer( $dom );
 		$actual    = $this->call_private_method( $sanitizer, 'xpath_from_css_selector', [ $css_selector ] );
 		$this->assertEquals( $expected, $actual );
 	}
 
 	public function get_get_closest_submenu_data() {
-		$html  = '
+		$html = '
 			<nav>
 				<ul class="primary-menu">
 					<li id="menu-item-1" class="menu-item menu-item-1"><a href="https://example.com/a">Link A</a></li>
@@ -71,17 +75,16 @@ public function get_get_closest_submenu_data() {
 				</ul>
 			</nav>
 		';
-		$dom   = AMP_DOM_Utils::get_dom_from_content( $html );
-		$xpath = new DOMXPath( $dom );
+		$dom  = AMP_DOM_Utils::get_dom_from_content( $html );
 		return [
 			// First sub-menu.
-			[ $dom, $xpath, $xpath->query( "//*[ @id = 'menu-item-2' ]" )->item( 0 ), $xpath->query( "//*[ @id = 'sub-menu-1' ]" )->item( 0 ) ],
+			[ $dom, $dom->xpath->query( "//*[ @id = 'menu-item-2' ]" )->item( 0 ), $dom->xpath->query( "//*[ @id = 'sub-menu-1' ]" )->item( 0 ) ],
 
 			// Second sub-menu.
-			[ $dom, $xpath, $xpath->query( "//*[ @id = 'menu-item-5' ]" )->item( 0 ), $xpath->query( "//*[ @id = 'sub-menu-2' ]" )->item( 0 ) ],
+			[ $dom, $dom->xpath->query( "//*[ @id = 'menu-item-5' ]" )->item( 0 ), $dom->xpath->query( "//*[ @id = 'sub-menu-2' ]" )->item( 0 ) ],
 
 			// Sub-menu of second sub-menu.
-			[ $dom, $xpath, $xpath->query( "//*[ @id = 'menu-item-6' ]" )->item( 0 ), $xpath->query( "//*[ @id = 'sub-menu-3' ]" )->item( 0 ) ],
+			[ $dom, $dom->xpath->query( "//*[ @id = 'menu-item-6' ]" )->item( 0 ), $dom->xpath->query( "//*[ @id = 'sub-menu-3' ]" )->item( 0 ) ],
 		];
 	}
 
@@ -90,11 +93,14 @@ public function get_get_closest_submenu_data() {
 	 *
 	 * @dataProvider get_get_closest_submenu_data
 	 * @covers AMP_Core_Theme_Sanitizer::get_closest_submenu()
+	 *
+	 * @param Document   $dom      Document.
+	 * @param DOMElement $element  Element.
+	 * @param DOMElement $expected Expected element.
 	 */
-	public function test_get_closest_submenu( $dom, $xpath, $element, $expected ) {
+	public function test_get_closest_submenu( $dom, $element, $expected ) {
 		$sanitizer = new AMP_Core_Theme_Sanitizer( $dom );
-		$this->set_private_property( $sanitizer, 'xpath', $xpath );
-		$actual = $this->call_private_method( $sanitizer, 'get_closest_submenu', [ $element ] );
+		$actual    = $this->call_private_method( $sanitizer, 'get_closest_submenu', [ $element ] );
 		$this->assertEquals( $expected, $actual );
 	}
 }
diff --git a/tests/php/test-class-amp-dom-document.php b/tests/php/test-class-amp-dom-document.php
new file mode 100644
index 00000000000..41a06f04b84
--- /dev/null
+++ b/tests/php/test-class-amp-dom-document.php
@@ -0,0 +1,449 @@
+<?php
+/**
+ * Tests for Amp\AmpWP\Dom\Document.
+ *
+ * @package AMP
+ */
+
+use Amp\AmpWP\Dom\Document;
+
+/**
+ * Tests for Amp\AmpWP\Dom\Document.
+ *
+ * @covers Document
+ */
+class Test_AMP_DOM_Document extends WP_UnitTestCase {
+
+	/**
+	 * Data for Amp\AmpWP\Dom\Document test.
+	 *
+	 * @return array Data.
+	 */
+	public function data_dom_document() {
+		$head = '<head><meta charset="utf-8"></head>';
+
+		return [
+			'minimum_valid_document'                   => [
+				'utf-8',
+				'<!DOCTYPE html><html><head></head><body></body></html>',
+				'<!DOCTYPE html><html>' . $head . '<body></body></html>',
+			],
+			'valid_document_with_attributes'           => [
+				'utf-8',
+				'<!DOCTYPE html><html amp lang="en">' . $head . '<body class="some-class"><p>Text</p></body></html>',
+				'<!DOCTYPE html><html amp lang="en">' . $head . '<body class="some-class"><p>Text</p></body></html>',
+			],
+			'missing_head'                             => [
+				'utf-8',
+				'<!DOCTYPE html><html amp lang="en"><body class="some-class"><p>Text</p></body></html>',
+				'<!DOCTYPE html><html amp lang="en">' . $head . '<body class="some-class"><p>Text</p></body></html>',
+			],
+			'missing_body'                             => [
+				'utf-8',
+				'<!DOCTYPE html><html amp lang="en">' . $head . '<p>Text</p></html>',
+				'<!DOCTYPE html><html amp lang="en">' . $head . '<body><p>Text</p></body></html>',
+			],
+			'missing_head_and_body'                    => [
+				'utf-8',
+				'<!DOCTYPE html><html amp lang="en"><p>Text</p></html>',
+				'<!DOCTYPE html><html amp lang="en">' . $head . '<body><p>Text</p></body></html>',
+			],
+			'missing_html_and_head_and_body'           => [
+				'utf-8',
+				'<!DOCTYPE html><p>Text</p>',
+				'<!DOCTYPE html><html>' . $head . '<body><p>Text</p></body></html>',
+			],
+			'content_only'                             => [
+				'utf-8',
+				'<p>Text</p>',
+				'<!DOCTYPE html><html>' . $head . '<body><p>Text</p></body></html>',
+			],
+			'missing_doctype'                          => [
+				'utf-8',
+				'<html amp lang="en">' . $head . '<body class="some-class"><p>Text</p></body></html>',
+				'<!DOCTYPE html><html amp lang="en">' . $head . '<body class="some-class"><p>Text</p></body></html>',
+			],
+			'html_4_doctype'                           => [
+				'utf-8',
+				'<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd"><html amp lang="en">' . $head . '<body class="some-class"><p>Text</p></body></html>',
+				'<!DOCTYPE html><html amp lang="en">' . $head . '<body class="some-class"><p>Text</p></body></html>',
+			],
+			'lots_of_whitespace'                       => [
+				'utf-8',
+				" \n <!DOCTYPE \n html \n > \n <html \n amp \n lang=\"en\"   \n  >  \n   <head >   \n<meta \n   charset=\"utf-8\">  \n  </head>  \n  <body   \n class=\"some-class\"  \n >  \n  <p>  \n  Text  \n  </p>  \n\n  </body  >  \n  </html  >  \n  ",
+				'<!DOCTYPE html><html amp lang="en">' . $head . '<body class="some-class"><p> Text </p></body></html>',
+			],
+			'utf_8_encoding_predefined'                => [
+				'utf-8',
+				'<p>مرحبا بالعالم! Check out ‘this’ and “that” and—other things.</p>',
+				'<!DOCTYPE html><html>' . $head . '<body><p>مرحبا بالعالم! Check out ‘this’ and “that” and—other things.</p></body></html>',
+			],
+			'utf_8_encoding_guessed_via_charset'       => [
+				'',
+				'<html>' . $head . '<body><p>مرحبا بالعالم! Check out ‘this’ and “that” and—other things.</p></body>',
+				'<!DOCTYPE html><html>' . $head . '<body><p>مرحبا بالعالم! Check out ‘this’ and “that” and—other things.</p></body></html>',
+			],
+			'utf_8_encoding_guessed_via_content'       => [
+				'',
+				'<p>مرحبا بالعالم! Check out ‘this’ and “that” and—other things.</p>',
+				'<!DOCTYPE html><html>' . $head . '<body><p>مرحبا بالعالم! Check out ‘this’ and “that” and—other things.</p></body></html>',
+			],
+			'iso_8859_1_encoding_predefined'           => [
+				'iso-8859-1',
+				utf8_decode( '<!DOCTYPE html><html><head></head><body><p>ÄÖÜ</p></body></html>' ),
+				'<!DOCTYPE html><html>' . $head . '<body><p>ÄÖÜ</p></body></html>',
+			],
+			'iso_8859_1_encoding_guessed_via_charset'  => [
+				'',
+				utf8_decode( '<!DOCTYPE html><html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /></head><body><p>ÄÖÜ</p></body></html>' ),
+				'<!DOCTYPE html><html>' . $head . '<body><p>ÄÖÜ</p></body></html>',
+			],
+			'iso_8859_1_encoding_guessed_via_content'  => [
+				'',
+				utf8_decode( '<!DOCTYPE html><html><body><p>ÄÖÜ</p></body></html>' ),
+				'<!DOCTYPE html><html>' . $head . '<body><p>ÄÖÜ</p></body></html>',
+			],
+			'raw_iso_8859_1'                           => [
+				'',
+				utf8_decode( 'ÄÖÜ' ),
+				'<!DOCTYPE html><html>' . $head . '<body>ÄÖÜ</body></html>',
+			],
+			// Make sure we correctly identify the ISO-8859 sub-charsets ("€" does not exist in ISO-8859-1).
+			'iso_8859_15_encoding_predefined'          => [
+				'iso-8859-1',
+				mb_convert_encoding( '<!DOCTYPE html><html><head></head><body><p>€</p></body></html>', 'ISO-8859-15', 'UTF-8' ),
+				'<!DOCTYPE html><html>' . $head . '<body><p>€</p></body></html>',
+			],
+			'iso_8859_15_encoding_guessed_via_charset' => [
+				'',
+				mb_convert_encoding( '<!DOCTYPE html><html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-15" /></head><body><p>€</p></body></html>', 'ISO-8859-15', 'UTF-8' ),
+				'<!DOCTYPE html><html>' . $head . '<body><p>€</p></body></html>',
+			],
+			'iso_8859_15_encoding_guessed_via_content' => [
+				'',
+				mb_convert_encoding( '<!DOCTYPE html><html><body><p>€</p></body></html>', 'ISO-8859-15', 'UTF-8' ),
+				'<!DOCTYPE html><html>' . $head . '<body><p>€</p></body></html>',
+			],
+			'raw_iso_8859_15'                          => [
+				'',
+				mb_convert_encoding( '€', 'ISO-8859-15', 'UTF-8' ),
+				'<!DOCTYPE html><html>' . $head . '<body>€</body></html>',
+			],
+		];
+	}
+
+	/**
+	 * Tests loading and saving the content via Amp\AmpWP\Dom\Document.
+	 *
+	 * @param string $charset  Charset to use.
+	 * @param string $source   Source content.
+	 * @param string $expected Expected target content.
+	 *
+	 * @dataProvider data_dom_document
+	 * @covers Document::loadHTML()
+	 * @covers Document::saveHTML()
+	 */
+	public function test_dom_document( $charset, $source, $expected ) {
+		$document = Document::from_html( $source, $charset );
+		$this->assertEqualMarkup( $expected, $document->saveHTML() );
+	}
+
+	/**
+	 * Assert markup is equal.
+	 *
+	 * @param string $expected Expected markup.
+	 * @param string $actual   Actual markup.
+	 */
+	public function assertEqualMarkup( $expected, $actual ) {
+		$actual   = preg_replace( '/\s+/', ' ', $actual );
+		$expected = preg_replace( '/\s+/', ' ', $expected );
+		$actual   = preg_replace( '/(?<=>)\s+(?=<)/', '', trim( $actual ) );
+		$expected = preg_replace( '/(?<=>)\s+(?=<)/', '', trim( $expected ) );
+
+		$this->assertEquals(
+			array_filter( preg_split( '#(<[^>]+>|[^<>]+)#', $expected, -1, PREG_SPLIT_DELIM_CAPTURE ) ),
+			array_filter( preg_split( '#(<[^>]+>|[^<>]+)#', $actual, -1, PREG_SPLIT_DELIM_CAPTURE ) )
+		);
+	}
+
+	/**
+	 * Test convert_amp_bind_attributes.
+	 *
+	 * @covers Document::convert_amp_bind_attributes()
+	 */
+	public function test_amp_bind_conversion() {
+		$original  = '<amp-img width=300 height="200" data-foo="bar" selected src="/img/dog.jpg" [src]="myAnimals[currentAnimal].imageUrl"></amp-img>';
+		$converted = Document::from_html( $original )->saveHTML();
+		$this->assertNotEquals( $original, $converted );
+		$this->assertContains( Document::AMP_BIND_DATA_ATTR_PREFIX . 'src="myAnimals[currentAnimal].imageUrl"', $converted );
+		$this->assertContains( 'width="300" height="200" data-foo="bar" selected', $converted );
+
+		// Check tag with self-closing attribute.
+		$original  = '<input type="text" role="textbox" class="calc-input" id="liens" name="liens" [value]="(result1 != null) ? result1.liens : \'verifying…\'" />';
+		$converted = Document::from_html( $original )->saveHTML();
+		$this->assertNotEquals( $original, $converted );
+
+		// Preserve trailing slash that is actually the attribute value.
+		$original  = '<a href=/>Home</a>';
+		$dom       = Document::from_html( $original );
+		$converted = $dom->saveHTML( $dom->body->firstChild );
+		$this->assertEquals( '<a href="/">Home</a>', $converted );
+
+		// Test malformed.
+		$malformed_html = [
+			'<amp-img width="123" [text]="..."</amp-img>',
+			'<amp-img width="123" [text="..."]></amp-img>',
+			'<amp-img width="123" [text]="..." *bad*></amp-img>',
+		];
+		foreach ( $malformed_html as $html ) {
+			$converted = Document::from_html( $html )->saveHTML();
+			$this->assertNotContains( Document::AMP_BIND_DATA_ATTR_PREFIX, $converted, "Source: {$html}" );
+		}
+	}
+
+	/**
+	 * Get Table Row Iterations
+	 *
+	 * @return array An array of arrays holding an integer representation of iterations.
+	 */
+	public function get_table_row_iterations() {
+		return [
+			[ 1 ],
+			[ 10 ],
+			[ 100 ],
+			[ 1000 ],
+			[ 10000 ],
+			[ 100000 ],
+		];
+	}
+
+	/**
+	 * Tests attribute conversions on content with iframe srcdocs of variable lengths.
+	 *
+	 * @dataProvider get_table_row_iterations
+	 *
+	 * @param int $iterations The number of table rows to append to iframe srcdoc.
+	 */
+	public function test_attribute_conversion_on_long_iframe_srcdocs( $iterations ) {
+		$html = '<html amp><head><meta charset="utf-8"></head><body><table>';
+
+		for ( $i = 0; $i < $iterations; $i++ ) {
+			$html .= '
+				<tr>
+				<td class="rank" style="width:2%;">1453</td>
+				<td class="text" style="width:10%;">1947</td>
+				<td class="text">Pittsburgh Ironmen</td>
+				<td class="boolean" style="width:10%;text-align:center;"></td>
+				<td class="number" style="width:10%;">1242</td>
+				<td class="number">1192</td>
+				<td class="number">1111</td>
+				<td class="number highlight">1182</td>
+				</tr>
+			';
+		}
+
+		$html .= '</table></body></html>';
+
+		$to_convert = sprintf(
+			'<amp-iframe sandbox="allow-scripts" srcdoc="%s"> </amp-iframe>',
+			htmlentities( $html )
+		);
+
+		Document::from_html( $to_convert )->saveHTML();
+
+		$this->assertSame( PREG_NO_ERROR, preg_last_error(), 'Probably failed when backtrack limit was exhausted.' );
+	}
+
+	/**
+	 * Test that HEAD and BODY elements are always present.
+	 *
+	 * @covers Document::normalize_document_structure()
+	 */
+	public function test_ensuring_head_body() {
+		// The meta charset tag that is automatically added needs to always be taken into account.
+
+		$html = '<html><body><p>Hello</p></body></html>';
+		$dom  = Document::from_html( $html );
+		$this->assertEquals( 'head', $dom->documentElement->firstChild->nodeName );
+		$this->assertEquals( 1, $dom->head->childNodes->length );
+		$this->assertEquals( 'body', $dom->documentElement->lastChild->nodeName );
+		$this->assertEquals( $dom->body, $dom->getElementsByTagName( 'p' )->item( 0 )->parentNode );
+
+		$html = '<html><head><title>foo</title></head></html>';
+		$dom  = Document::from_html( $html );
+		$this->assertEquals( 'head', $dom->documentElement->firstChild->nodeName );
+		$this->assertEquals( $dom->head->firstChild, $dom->getElementsByTagName( 'meta' )->item( 0 ) );
+		$this->assertEquals( $dom->head->firstChild->nextSibling, $dom->getElementsByTagName( 'title' )->item( 0 ) );
+		$this->assertEquals( 'body', $dom->documentElement->lastChild->nodeName );
+		$this->assertEquals( 0, $dom->body->childNodes->length );
+
+		$html = '<html><head><title>foo</title></head><p>no body</p></html>';
+		$dom  = Document::from_html( $html );
+		$this->assertEquals( 'head', $dom->documentElement->firstChild->nodeName );
+		$this->assertEquals( $dom->head->firstChild, $dom->getElementsByTagName( 'meta' )->item( 0 ) );
+		$this->assertEquals( $dom->head->firstChild->nextSibling, $dom->getElementsByTagName( 'title' )->item( 0 ) );
+		$p = $dom->getElementsByTagName( 'p' )->item( 0 );
+		$this->assertEquals( $dom->body, $p->parentNode );
+		$this->assertEquals( 'no body', $p->textContent );
+
+		$html = 'Hello world';
+		$dom  = Document::from_html( $html );
+		$this->assertEquals( 'head', $dom->documentElement->firstChild->nodeName );
+		$this->assertEquals( 1, $dom->head->childNodes->length );
+		$this->assertEquals( 'body', $dom->documentElement->lastChild->nodeName );
+		$this->assertEquals( 'Hello world', $dom->body->lastChild->textContent );
+	}
+
+	/**
+	 * Test that invalid head nodes are moved to body.
+	 *
+	 * @covers Document::move_invalid_head_nodes_to_body()
+	 */
+	public function test_invalid_head_nodes() {
+		// The meta charset tag that is automatically added needs to always be taken into account.
+
+		// Text node.
+		$html = '<html><head>text</head><body><span>end</span></body></html>';
+		$dom  = Document::from_html( $html );
+		$this->assertEquals( 'meta', $dom->head->firstChild->tagName );
+		$this->assertNull( $dom->head->firstChild->nextSibling );
+		$body_first_child = $dom->body->firstChild;
+		$this->assertInstanceOf( 'DOMElement', $body_first_child );
+		$this->assertEquals( 'text', $body_first_child->textContent );
+
+		// Valid nodes.
+		$html = '<html><head><!--foo--><title>a</title><base href="/"><meta name="foo" content="bar"><link rel="test" href="/"><style></style><noscript><img src="http://example.com/foo.png"></noscript><script></script></head><body></body></html>';
+		$dom  = Document::from_html( $html );
+		$this->assertEquals( 9, $dom->head->childNodes->length );
+		$this->assertNull( $dom->body->firstChild );
+
+		// Invalid nodes.
+		$html = '<html><head><?pi ?><span></span><div></div><p>hi</p><img src="https://example.com"><iframe src="/"></iframe></head><body></body></html>';
+		$dom  = Document::from_html( $html );
+		$this->assertEquals( 'meta', $dom->head->firstChild->tagName );
+		$this->assertNull( $dom->head->firstChild->nextSibling );
+		$this->assertEquals( 6, $dom->body->childNodes->length );
+	}
+
+	/**
+	 * Get head node data.
+	 *
+	 * @return array Head node data.
+	 */
+	public function get_head_node_data() {
+		$dom = new Document();
+		return [
+			[
+				$dom,
+				AMP_DOM_Utils::create_node( $dom, 'title', [] ),
+				true,
+			],
+			[
+				$dom,
+				AMP_DOM_Utils::create_node(
+					$dom,
+					'base',
+					[ 'href' => '/' ]
+				),
+				true,
+			],
+			[
+				$dom,
+				AMP_DOM_Utils::create_node(
+					$dom,
+					'script',
+					[ 'src' => 'http://example.com/test.js' ]
+				),
+				true,
+			],
+			[
+				$dom,
+				AMP_DOM_Utils::create_node( $dom, 'style', [ 'media' => 'print' ] ),
+				true,
+			],
+			[
+				$dom,
+				AMP_DOM_Utils::create_node( $dom, 'noscript', [] ),
+				true,
+			],
+			[
+				$dom,
+				AMP_DOM_Utils::create_node(
+					$dom,
+					'link',
+					[
+						'rel'  => 'stylesheet',
+						'href' => 'https://example.com/foo.css',
+					]
+				),
+				true,
+			],
+			[
+				$dom,
+				AMP_DOM_Utils::create_node(
+					$dom,
+					'meta',
+					[
+						'name'    => 'foo',
+						'content' => 'https://example.com/foo.css',
+					]
+				),
+				true,
+			],
+			[
+				$dom,
+				$dom->createTextNode( " \n\t" ),
+				true,
+			],
+			[
+				$dom,
+				$dom->createTextNode( 'no' ),
+				false,
+			],
+			[
+				$dom,
+				$dom->createComment( 'hello world' ),
+				true,
+			],
+			[
+				$dom,
+				$dom->createProcessingInstruction( 'test' ),
+				false,
+			],
+			[
+				$dom,
+				$dom->createCDATASection( 'nope' ),
+				false,
+			],
+			[
+				$dom,
+				$dom->createEntityReference( 'bad' ),
+				false,
+			],
+			[
+				$dom,
+				$dom->createElementNS( 'http://www.w3.org/2000/svg', 'svg' ),
+				false,
+			],
+			[
+				$dom,
+				AMP_DOM_Utils::create_node( $dom, 'span', [] ),
+				false,
+			],
+		];
+	}
+
+	/**
+	 * Test is_valid_head_node().
+	 *
+	 * @dataProvider get_head_node_data
+	 * @covers       Document::is_valid_head_node()
+	 *
+	 * @param Document $dom   DOM document to use.
+	 * @param DOMNode  $node  Node.
+	 * @param bool     $valid Expected valid.
+	 */
+	public function test_is_valid_head_node( $dom, $node, $valid ) {
+		$this->assertEquals( $valid, $dom->is_valid_head_node( $node ) );
+	}
+}
diff --git a/tests/php/test-class-amp-dom-utils.php b/tests/php/test-class-amp-dom-utils.php
index 99999974a90..f69cfbb6c54 100644
--- a/tests/php/test-class-amp-dom-utils.php
+++ b/tests/php/test-class-amp-dom-utils.php
@@ -1,5 +1,7 @@
 <?php
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Class AMP_DOM_Utils_Test
  *
@@ -158,43 +160,10 @@ public function test__get_content_from_dom__br_no_closing_tag() {
 		$this->assertEquals( $expected, $actual );
 	}
 
-	/**
-	 * Test convert_amp_bind_attributes.
-	 *
-	 * @covers \AMP_DOM_Utils::convert_amp_bind_attributes()
-	 */
-	public function test_amp_bind_conversion() {
-		$original  = '<amp-img width=300 height="200" data-foo="bar" selected src="/img/dog.jpg" [src]="myAnimals[currentAnimal].imageUrl"></amp-img>';
-		$converted = AMP_DOM_Utils::convert_amp_bind_attributes( $original );
-		$this->assertNotEquals( $converted, $original );
-		$this->assertContains( AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX . 'src="myAnimals[currentAnimal].imageUrl"', $converted );
-		$this->assertContains( 'width=300 height="200" data-foo="bar" selected', $converted );
-
-		// Check tag with self-closing attribute.
-		$original  = '<input type="text" role="textbox" class="calc-input" id="liens" name="liens" [value]="(result1 != null) ? result1.liens : \'verifying…\'" />';
-		$converted = AMP_DOM_Utils::convert_amp_bind_attributes( $original );
-		$this->assertNotEquals( $converted, $original );
-
-		// Preserve trailing slash that is actually the attribute value.
-		$original = '<a href=/>Home</a>';
-		$this->assertEquals( AMP_DOM_Utils::convert_amp_bind_attributes( $original ), $original );
-
-		// Test malformed.
-		$malformed_html = [
-			'<amp-img width="123" [text]="..."</amp-img>',
-			'<amp-img width="123" [text="..."]></amp-img>',
-			'<amp-img width="123" [text]="..." *bad*></amp-img>',
-		];
-		foreach ( $malformed_html as $html ) {
-			$converted = AMP_DOM_Utils::convert_amp_bind_attributes( $html );
-			$this->assertNotContains( AMP_DOM_Utils::AMP_BIND_DATA_ATTR_PREFIX, $converted, "Source: $html" );
-		}
-	}
-
 	/**
 	 * Test handling of empty elements.
 	 *
-	 * @covers \AMP_DOM_Utils::get_dom()
+	 * @covers \Amp\AmpWP\Dom\Document::from_html()
 	 * @covers \AMP_DOM_Utils::get_content_from_dom_node()
 	 */
 	public function test_html5_empty_elements() {
@@ -222,7 +191,7 @@ public function test_html5_empty_elements() {
 	/**
 	 * Test parsing DOM with Mustache or Mustache-like templates.
 	 *
-	 * @covers \AMP_DOM_Utils::get_dom()
+	 * @covers \Amp\AmpWP\Dom\Document::from_html()
 	 * @covers \AMP_DOM_Utils::get_content_from_dom_node()
 	 */
 	public function test_mustache_replacements() {
@@ -255,11 +224,10 @@ public function test_mustache_replacements() {
 			]
 		);
 
-		$dom   = AMP_DOM_Utils::get_dom_from_content( $html );
-		$xpath = new DOMXPath( $dom );
+		$dom = AMP_DOM_Utils::get_dom_from_content( $html );
 
 		// Ensure that JSON in scripts are left intact.
-		$script = $xpath->query( '//script' )->item( 0 );
+		$script = $dom->xpath->query( '//script' )->item( 0 );
 		$this->assertEquals(
 			$data,
 			json_decode( $script->nodeValue, true )
@@ -273,19 +241,19 @@ public function test_mustache_replacements() {
 		 * @var DOMElement $template_img
 		 * @var DOMElement $template_blockquote
 		 */
-		$template_link = $xpath->query( '//template/a' )->item( 0 );
+		$template_link = $dom->xpath->query( '//template/a' )->item( 0 );
 		$this->assertSame( '{{href}}', $template_link->getAttribute( 'href' ) );
 		$this->assertEquals( 'Hello {{name}}', $template_link->getAttribute( 'title' ) );
 
 		// Ensure that mustache var in img[src] attribute is intact.
-		$template_img = $xpath->query( '//template/a/img' )->item( 0 );
+		$template_img = $dom->xpath->query( '//template/a/img' )->item( 0 );
 		$this->assertEquals( '{{src}}', $template_img->getAttribute( 'src' ) );
 
 		// Ensure that mustache var in blockquote[cite] is not changed.
-		$template_blockquote = $xpath->query( '//template/blockquote' )->item( 0 );
+		$template_blockquote = $dom->xpath->query( '//template/blockquote' )->item( 0 );
 		$this->assertEquals( '{{cite}}', $template_blockquote->getAttribute( 'cite' ) );
 
-		$serialized_html = AMP_DOM_Utils::get_content_from_dom_node( $dom, $dom->documentElement );
+		$serialized_html = $dom->saveHTML( $dom->documentElement );
 
 		$this->assertContains( '<a href="{{href}}" title="Hello {{name}}">', $serialized_html );
 		$this->assertContains( '<img src="{{src}}">', $serialized_html );
@@ -296,18 +264,18 @@ public function test_mustache_replacements() {
 	/**
 	 * Test encoding.
 	 *
-	 * @covers \AMP_DOM_Utils::get_dom()
+	 * @covers \Amp\AmpWP\Dom\Document::from_html()
 	 */
 	public function test_get_dom_encoding() {
-		$html  = '<!DOCTYPE html><html><head><title>مرحبا بالعالم! Check out ‘this’ and “that” and—other things.</title><meta charset="UTF-8"></head><body>';
+		$html  = '<!DOCTYPE html><html><head><title>مرحبا بالعالم! Check out ‘this’ and “that” and—other things.</title><meta charset="utf-8"></head><body>';
 		$html .= '<p>مرحبا بالعالم! Check out ‘this’ and “that” and—other things.</p>';
 		$html .= '<p>&#x645;&#x631;&#x62D;&#x628;&#x627; &#x628;&#x627;&#x644;&#x639;&#x627;&#x644;&#x645;! Check out &#8216;this&#8217; and &#8220;that&#8221; and&#8212;other things.</p>';
 		$html .= '<p>&#x645;&#x631;&#x62D;&#x628;&#x627; &#x628;&#x627;&#x644;&#x639;&#x627;&#x644;&#x645;! Check out &lsquo;this&rsquo; and &ldquo;that&rdquo; and&mdash;other things.</p>';
 		$html .= '</body></html>';
 
-		$document = AMP_DOM_Utils::get_dom( $html );
+		$document = Document::from_html( $html );
 
-		$this->assertEquals( 'UTF-8', $document->encoding );
+		$this->assertEquals( 'utf-8', $document->encoding );
 		$paragraphs = $document->getElementsByTagName( 'p' );
 		$this->assertSame( 3, $paragraphs->length );
 		$this->assertSame( $paragraphs->item( 0 )->textContent, $paragraphs->item( 1 )->textContent );
@@ -316,60 +284,7 @@ public function test_get_dom_encoding() {
 	}
 
 	/**
-	 * Get Table Row Iterations
-	 *
-	 * @return array An array of arrays holding an integer representation of iterations.
-	 */
-	public function get_table_row_iterations() {
-		return [
-			[ 1 ],
-			[ 10 ],
-			[ 100 ],
-			[ 1000 ],
-			[ 10000 ],
-			[ 100000 ],
-		];
-	}
-
-	/**
-	 * Tests attribute conversions on content with iframe srcdocs of variable lengths.
-	 *
-	 * @dataProvider get_table_row_iterations
-	 *
-	 * @param int $iterations The number of table rows to append to iframe srcdoc.
-	 */
-	public function test_attribute_conversion_on_long_iframe_srcdocs( $iterations ) {
-		$html = '<html amp><head><meta charset="utf-8"></head><body><table>';
-
-		for ( $i = 0; $i < $iterations; $i++ ) {
-			$html .= '
-				<tr>
-				<td class="rank" style="width:2%;">1453</td>
-				<td class="text" style="width:10%;">1947</td>
-				<td class="text">Pittsburgh Ironmen</td>
-				<td class="boolean" style="width:10%;text-align:center;"></td>
-				<td class="number" style="width:10%;">1242</td>
-				<td class="number">1192</td>
-				<td class="number">1111</td>
-				<td class="number highlight">1182</td>
-				</tr>
-			';
-		}
-
-		$html .= '</table></body></html>';
-
-		$to_convert = sprintf(
-			'<amp-iframe sandbox="allow-scripts" srcdoc="%s"> </amp-iframe>',
-			htmlentities( $html )
-		);
-
-		AMP_DOM_Utils::convert_amp_bind_attributes( $to_convert );
-
-		$this->assertSame( PREG_NO_ERROR, preg_last_error(), 'Probably failed when backtrack limit was exhausted.' );
-	}
-
-	/**
-	 * Test preserving whitespace when serializing DOMDocument as HTML string.
+	 * Test preserving whitespace when serializing Dom\Document as HTML string.
 	 *
 	 * @covers \AMP_DOM_Utils::get_content_from_dom_node()
 	 * @covers \AMP_DOM_Utils::get_content_from_dom()
@@ -379,189 +294,17 @@ public function test_whitespace_preservation() {
 		$body = " start <ul><li>First</li><li>Second</li></ul><style>pre::before { content:'⚡️'; }</style><script type=\"application/json\">\"⚡️\"</script><pre>\t* one\n\t* two\n\t* three</pre> end ";
 		$html = "<html><head><meta charset=\"utf-8\"></head><body data-foo=\"&gt;\">$body</body></html>";
 
-		$dom = AMP_DOM_Utils::get_dom( "<!DOCTYPE html>$html" );
+		$dom = Document::from_html( "<!DOCTYPE html>$html" );
 
-		$output = AMP_DOM_Utils::get_content_from_dom_node( $dom, $dom->documentElement );
+		$output = $dom->saveHTML( $dom->documentElement );
 		$this->assertEquals( $html, $output );
 
 		$output = AMP_DOM_Utils::get_content_from_dom( $dom );
 		$this->assertEquals( $body, $output );
 	}
 
-	/**
-	 * Test that HEAD and BODY elements are always present.
-	 *
-	 * @covers \AMP_DOM_Utils::get_dom()
-	 */
-	public function test_ensuring_head_body() {
-		$html = '<html><body><p>Hello</p></body></html>';
-		$dom  = AMP_DOM_Utils::get_dom( $html );
-		$this->assertEquals( 'head', $dom->documentElement->firstChild->nodeName );
-		$this->assertEquals( 0, $dom->documentElement->firstChild->childNodes->length );
-		$this->assertEquals( 'body', $dom->documentElement->lastChild->nodeName );
-		$this->assertEquals( $dom->documentElement->lastChild, $dom->getElementsByTagName( 'p' )->item( 0 )->parentNode );
-
-		$html = '<html><head><title>foo</title></head></html>';
-		$dom  = AMP_DOM_Utils::get_dom( $html );
-		$this->assertEquals( 'head', $dom->documentElement->firstChild->nodeName );
-		$this->assertEquals( $dom->documentElement->firstChild, $dom->getElementsByTagName( 'title' )->item( 0 )->parentNode );
-		$this->assertEquals( 'body', $dom->documentElement->lastChild->nodeName );
-		$this->assertEquals( 0, $dom->documentElement->lastChild->childNodes->length );
-
-		$html = '<html><head><title>foo</title></head><p>no body</p></html>';
-		$dom  = AMP_DOM_Utils::get_dom( $html );
-		$this->assertEquals( 'head', $dom->documentElement->firstChild->nodeName );
-		$this->assertEquals( $dom->documentElement->firstChild, $dom->getElementsByTagName( 'title' )->item( 0 )->parentNode );
-		$p = $dom->getElementsByTagName( 'p' )->item( 0 );
-		$this->assertEquals( $dom->documentElement->lastChild, $p->parentNode );
-		$this->assertEquals( 'no body', $p->textContent );
-
-		$html = 'Hello world';
-		$dom  = AMP_DOM_Utils::get_dom( $html );
-		$this->assertEquals( 'head', $dom->documentElement->firstChild->nodeName );
-		$this->assertEquals( 0, $dom->documentElement->firstChild->childNodes->length );
-		$this->assertEquals( 'body', $dom->documentElement->lastChild->nodeName );
-		$p = $dom->getElementsByTagName( 'p' )->item( 0 );
-		$this->assertEquals( $dom->documentElement->lastChild, $p->parentNode );
-		$this->assertEquals( 'Hello world', $p->textContent );
-	}
-
-	/**
-	 * Get head node data.
-	 *
-	 * @return array Head node data.
-	 */
-	public function get_head_node_data() {
-		$dom = new DOMDocument();
-		return [
-			[
-				AMP_DOM_Utils::create_node( $dom, 'title', [] ),
-				true,
-			],
-			[
-				AMP_DOM_Utils::create_node(
-					$dom,
-					'base',
-					[ 'href' => '/' ]
-				),
-				true,
-			],
-			[
-				AMP_DOM_Utils::create_node(
-					$dom,
-					'script',
-					[ 'src' => 'http://example.com/test.js' ]
-				),
-				true,
-			],
-			[
-				AMP_DOM_Utils::create_node( $dom, 'style', [ 'media' => 'print' ] ),
-				true,
-			],
-			[
-				AMP_DOM_Utils::create_node( $dom, 'noscript', [] ),
-				true,
-			],
-			[
-				AMP_DOM_Utils::create_node(
-					$dom,
-					'link',
-					[
-						'rel'  => 'stylesheet',
-						'href' => 'https://example.com/foo.css',
-					]
-				),
-				true,
-			],
-			[
-				AMP_DOM_Utils::create_node(
-					$dom,
-					'meta',
-					[
-						'name'    => 'foo',
-						'content' => 'https://example.com/foo.css',
-					]
-				),
-				true,
-			],
-			[
-				$dom->createTextNode( " \n\t" ),
-				true,
-			],
-			[
-				$dom->createTextNode( 'no' ),
-				false,
-			],
-			[
-				$dom->createComment( 'hello world' ),
-				true,
-			],
-			[
-				$dom->createProcessingInstruction( 'test' ),
-				false,
-			],
-			[
-				$dom->createCDATASection( 'nope' ),
-				false,
-			],
-			[
-				$dom->createEntityReference( 'bad' ),
-				false,
-			],
-			[
-				$dom->createElementNS( 'http://www.w3.org/2000/svg', 'svg' ),
-				false,
-			],
-			[
-				AMP_DOM_Utils::create_node( $dom, 'span', [] ),
-				false,
-			],
-		];
-	}
-
-	/**
-	 * Test is_valid_head_node().
-	 *
-	 * @dataProvider get_head_node_data
-	 * @covers \AMP_DOM_Utils::is_valid_head_node()
-	 *
-	 * @param DOMNode $node  Node.
-	 * @param bool    $valid Expected valid.
-	 */
-	public function test_is_valid_head_node( $node, $valid ) {
-		$this->assertEquals( $valid, AMP_DOM_Utils::is_valid_head_node( $node ) );
-	}
-
-	/**
-	 * Test that invalid head nodes are moved to body.
-	 *
-	 * @covers \AMP_DOM_Utils::move_invalid_head_nodes_to_body()
-	 */
-	public function test_invalid_head_nodes() {
-
-		// Text node.
-		$html = '<html><head>text</head><body><span>end</span></body></html>';
-		$dom  = AMP_DOM_Utils::get_dom( $html );
-		$this->assertNull( $dom->getElementsByTagName( 'head' )->item( 0 )->firstChild );
-		$body_first_child = $dom->getElementsByTagName( 'body' )->item( 0 )->firstChild;
-		$this->assertInstanceOf( 'DOMElement', $body_first_child );
-		$this->assertEquals( 'text', $body_first_child->textContent );
-
-		// Valid nodes.
-		$html = '<html><head><!--foo--><title>a</title><base href="/"><meta name="foo" content="bar"><link rel="test" href="/"><style></style><noscript><img src="http://example.com/foo.png"></noscript><script></script></head><body></body></html>';
-		$dom  = AMP_DOM_Utils::get_dom( $html );
-		$this->assertEquals( 8, $dom->getElementsByTagName( 'head' )->item( 0 )->childNodes->length );
-		$this->assertNull( $dom->getElementsByTagName( 'body' )->item( 0 )->firstChild );
-
-		// Invalid nodes.
-		$html = '<html><head><?pi ?><span></span><div></div><p>hi</p><img src="https://example.com"><iframe src="/"></iframe></head><body></body></html>';
-		$dom  = AMP_DOM_Utils::get_dom( $html );
-		$this->assertNull( $dom->getElementsByTagName( 'head' )->item( 0 )->firstChild );
-		$this->assertEquals( 6, $dom->getElementsByTagName( 'body' )->item( 0 )->childNodes->length );
-	}
-
 	public function get_has_class_data() {
-		$dom = new DOMDocument();
+		$dom = new Document();
 
 		return [
 			// Element without class attribute.
@@ -592,6 +335,10 @@ public function get_has_class_data() {
 	 *
 	 * @dataProvider get_has_class_data
 	 * @covers \AMP_DOM_Utils::has_class()
+	 *
+	 * @param DOMElement $element  Element.
+	 * @param string     $class    Class names.
+	 * @param bool       $expected Expected has class name.
 	 */
 	public function test_has_class( DOMElement $element, $class, $expected ) {
 		$actual = AMP_DOM_Utils::has_class( $element, $class );
@@ -599,7 +346,7 @@ public function test_has_class( DOMElement $element, $class, $expected ) {
 	}
 
 	public function get_get_element_id_data() {
-		$dom = new DOMDocument();
+		$dom = new Document();
 
 		$same_element = AMP_DOM_Utils::create_node( $dom, 'div', [] );
 
@@ -630,6 +377,10 @@ public function get_get_element_id_data() {
 	 *
 	 * @dataProvider get_get_element_id_data
 	 * @covers \AMP_DOM_Utils::get_element_id()
+	 *
+	 * @param DOMElement $element  Element.
+	 * @param string     $prefix   Prefix.
+	 * @param string     $expected Expected element ID.
 	 */
 	public function test_get_element_id( DOMElement $element, $prefix, $expected ) {
 		$actual = AMP_DOM_Utils::get_element_id( $element, $prefix );
@@ -637,7 +388,7 @@ public function test_get_element_id( DOMElement $element, $prefix, $expected ) {
 	}
 
 	public function get_add_amp_action_data() {
-		$dom    = new DOMDocument();
+		$dom    = new Document();
 		$button = AMP_DOM_Utils::create_node( $dom, 'button', [] );
 		$form   = AMP_DOM_Utils::create_node( $dom, 'form', [] );
 
@@ -668,6 +419,11 @@ public function get_add_amp_action_data() {
 	 *
 	 * @dataProvider get_add_amp_action_data
 	 * @covers \AMP_DOM_Utils::add_amp_action()
+	 *
+	 * @param DOMElement $element  Element.
+	 * @param string     $event    Event.
+	 * @param string     $action   Action.
+	 * @param string     $expected Expected.
 	 */
 	public function test_add_amp_action( DOMElement $element, $event, $action, $expected ) {
 		AMP_DOM_Utils::add_amp_action( $element, $event, $action );
@@ -703,6 +459,10 @@ public function get_merge_amp_actions_data() {
 	 *
 	 * @dataProvider get_merge_amp_actions_data
 	 * @covers \AMP_DOM_Utils::merge_amp_actions()
+	 *
+	 * @param string $first    First action.
+	 * @param string $second   Second action.
+	 * @param string $expected Expected merged actions.
 	 */
 	public function test_merge_amp_actions( $first, $second, $expected ) {
 		$actual = AMP_DOM_Utils::merge_amp_actions( $first, $second );
@@ -710,7 +470,7 @@ public function test_merge_amp_actions( $first, $second, $expected ) {
 	}
 
 	public function get_copy_attributes_data() {
-		$dom = new DOMDocument();
+		$dom = new Document();
 
 		return [
 			// No attributes from full to empty.
@@ -890,6 +650,11 @@ public function get_copy_attributes_data() {
 	 *
 	 * @dataProvider get_copy_attributes_data
 	 * @covers \AMP_DOM_Utils::copy_attributes()
+	 *
+	 * @param array      $attributes Attributes.
+	 * @param DOMElement $from       From element.
+	 * @param DOMElement $to         To element.
+	 * @param array      $expected   Expected.
 	 */
 	public function test_copy_attributes( $attributes, DOMElement $from, DOMElement $to, $expected ) {
 		AMP_DOM_Utils::copy_attributes( $attributes, $from, $to );
diff --git a/tests/php/test-class-amp-link-sanitizer.php b/tests/php/test-class-amp-link-sanitizer.php
index f1fed58531a..01cca805b5e 100644
--- a/tests/php/test-class-amp-link-sanitizer.php
+++ b/tests/php/test-class-amp-link-sanitizer.php
@@ -142,10 +142,9 @@ public function test_amp_to_amp_navigation( $paired ) {
 		}
 
 		// Confirm changes to form.
-		$xpath = new DOMXPath( $dom );
-		$this->assertEquals( 1, $xpath->query( '//form[ @id = "internal-search" ]//input[ @name = "amp" ]' )->length );
-		$this->assertEquals( 0, $xpath->query( '//form[ @id = "internal-post" ]//input[ @name = "amp" ]' )->length );
-		$this->assertEquals( 0, $xpath->query( '//form[ @id = "external-search" ]//input[ @name = "amp" ]' )->length );
+		$this->assertEquals( 1, $dom->xpath->query( '//form[ @id = "internal-search" ]//input[ @name = "amp" ]' )->length );
+		$this->assertEquals( 0, $dom->xpath->query( '//form[ @id = "internal-post" ]//input[ @name = "amp" ]' )->length );
+		$this->assertEquals( 0, $dom->xpath->query( '//form[ @id = "external-search" ]//input[ @name = "amp" ]' )->length );
 	}
 
 	/**
@@ -176,13 +175,12 @@ public function get_test_amp_to_amp_meta_tag_data() {
 	 * @param string $expected_meta Expected meta content.
 	 */
 	public function test_amp_to_amp_meta_tag( $sanitizer_args, $expected_meta ) {
-		$dom   = AMP_DOM_Utils::get_dom_from_content( '<div>Hello</div>' );
-		$xpath = new DOMXPath( $dom );
+		$dom = AMP_DOM_Utils::get_dom_from_content( '<div>Hello</div>' );
 
 		$sanitizer = new AMP_Link_Sanitizer( $dom, $sanitizer_args );
 		$sanitizer->sanitize();
 
-		$meta_tag = $xpath->query( "//meta[ @name = 'amp-to-amp-navigation' ]" )->item( 0 );
+		$meta_tag = $dom->xpath->query( "//meta[ @name = 'amp-to-amp-navigation' ]" )->item( 0 );
 		$this->assertInstanceOf( 'DOMElement', $meta_tag );
 		$this->assertEquals( $expected_meta, $meta_tag->getAttribute( 'content' ) );
 	}
diff --git a/tests/php/test-class-amp-meta-sanitizer.php b/tests/php/test-class-amp-meta-sanitizer.php
new file mode 100644
index 00000000000..0ea2c75af15
--- /dev/null
+++ b/tests/php/test-class-amp-meta-sanitizer.php
@@ -0,0 +1,110 @@
+<?php
+/**
+ * Tests for AMP_Meta_Sanitizer.
+ *
+ * @package AMP
+ */
+
+use Amp\AmpWP\Dom\Document;
+
+/**
+ * Tests for AMP_Meta_Sanitizer.
+ */
+class Test_AMP_Meta_Sanitizer extends WP_UnitTestCase {
+
+	/**
+	 * Provide data to the test_sanitize method.
+	 *
+	 * @return array[] Array of arrays with test data.
+	 */
+	public function get_data_for_sanitize() {
+		$script1 = 'document.body.textContent += "First!";';
+		$script2 = 'document.body.textContent += "Second!";';
+		$script3 = 'document.body.textContent += "Third!";';
+		$script4 = 'document.body.textContent += "Fourth! (And forbidden because no amp-script-src meta in head.)";';
+
+		$script1_hash = amp_generate_script_hash( $script1 );
+		$script2_hash = amp_generate_script_hash( $script2 );
+		$script3_hash = amp_generate_script_hash( $script3 );
+		$script4_hash = amp_generate_script_hash( $script4 );
+
+		return [
+			// Don't break the correct charset tag.
+			[
+				'<!DOCTYPE html><html><head><meta charset="utf-8"><meta name="viewport" content="width=device-width"></head><body></body></html>',
+				'<!DOCTYPE html><html><head><meta charset="utf-8"><meta name="viewport" content="width=device-width"></head><body></body></html>',
+			],
+
+			// Don't break the correct viewport tag.
+			[
+				'<!DOCTYPE html><html><head><meta charset="utf-8"><meta name="viewport" content="width=device-width,minimum-scale=1,initial-scale=1"></head><body></body></html>',
+				'<!DOCTYPE html><html><head><meta charset="utf-8"><meta name="viewport" content="width=device-width,minimum-scale=1,initial-scale=1"></head><body></body></html>',
+			],
+
+			// Move charset and viewport tags from body to head.
+			[
+				'<!DOCTYPE html><html><head></head><body><meta charset="utf-8"><meta name="viewport" content="width=device-width"></body></html>',
+				'<!DOCTYPE html><html><head><meta charset="utf-8"><meta name="viewport" content="width=device-width"></head><body></body></html>',
+			],
+
+			// Add default charset tag if none is present.
+			[
+				'<!DOCTYPE html><html><head><meta name="viewport" content="width=device-width"></head><body></body></html>',
+				'<!DOCTYPE html><html><head><meta charset="utf-8"><meta name="viewport" content="width=device-width"></head><body></body></html>',
+			],
+
+			// Add default viewport tag if none is present.
+			[
+				'<!DOCTYPE html><html><head><meta charset="utf-8"></head><body></body></html>',
+				'<!DOCTYPE html><html><head><meta charset="utf-8"><meta name="viewport" content="width=device-width"></head><body></body></html>',
+			],
+
+			// Make sure charset is the first meta tag.
+			[
+				'<!DOCTYPE html><html><head><meta name="viewport" content="width=device-width"><meta charset="utf-8"></head><body></body></html>',
+				'<!DOCTYPE html><html><head><meta charset="utf-8"><meta name="viewport" content="width=device-width"></head><body></body></html>',
+			],
+
+			// Concatenate and reposition script hashes.
+			[
+				'<!DOCTYPE html><html><head><meta name="amp-script-src" content="' . esc_attr( $script1_hash ) . '"><meta charset="utf-8"><meta name="amp-script-src" content="' . esc_attr( $script2_hash ) . '"><meta name="viewport" content="width=device-width"><meta name="amp-script-src" content="' . esc_attr( $script3_hash ) . '"></head><body><meta name="amp-script-src" content="' . esc_attr( $script4_hash ) . '"></body></html>',
+				'<!DOCTYPE html><html><head><meta charset="utf-8"><meta name="viewport" content="width=device-width"><meta name="amp-script-src" content="' . esc_attr( $script1_hash ) . ' ' . esc_attr( $script2_hash ) . ' ' . esc_attr( $script3_hash ) . ' ' . esc_attr( $script4_hash ) . '"></head><body></body></html>',
+			],
+		];
+	}
+
+	/**
+	 * Tests the sanitize method.
+	 *
+	 * @dataProvider get_data_for_sanitize
+	 * @covers \AMP_Meta_Sanitizer::sanitize()
+	 *
+	 * @param string  $source_content   Source DOM content.
+	 * @param string  $expected_content Expected content after sanitization.
+	 */
+	public function test_sanitize( $source_content, $expected_content ) {
+		$dom       = Document::from_html( $source_content );
+		$sanitizer = new AMP_Meta_Sanitizer( $dom );
+		$sanitizer->sanitize();
+
+		$this->assertEqualMarkup( $expected_content, $dom->saveHTML() );
+	}
+
+	/**
+	 * Assert markup is equal.
+	 *
+	 * @param string $expected Expected markup.
+	 * @param string $actual   Actual markup.
+	 */
+	public function assertEqualMarkup( $expected, $actual ) {
+		$actual   = preg_replace( '/\s+/', ' ', $actual );
+		$expected = preg_replace( '/\s+/', ' ', $expected );
+		$actual   = preg_replace( '/(?<=>)\s+(?=<)/', '', trim( $actual ) );
+		$expected = preg_replace( '/(?<=>)\s+(?=<)/', '', trim( $expected ) );
+
+		$this->assertEquals(
+			array_filter( preg_split( '#(<[^>]+>|[^<>]+)#', $expected, -1, PREG_SPLIT_DELIM_CAPTURE ) ),
+			array_filter( preg_split( '#(<[^>]+>|[^<>]+)#', $actual, -1, PREG_SPLIT_DELIM_CAPTURE ) )
+		);
+	}
+}
diff --git a/tests/php/test-class-amp-nav-menu-toggle-sanitizer.php b/tests/php/test-class-amp-nav-menu-toggle-sanitizer.php
index 5324f57d0dd..22c5e392550 100644
--- a/tests/php/test-class-amp-nav-menu-toggle-sanitizer.php
+++ b/tests/php/test-class-amp-nav-menu-toggle-sanitizer.php
@@ -5,11 +5,12 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Tests for AMP_Nav_Menu_Toggle_Sanitizer.
  *
  * @covers AMP_Nav_Menu_Toggle_Sanitizer
- * @group testtt
  */
 class Test_AMP_Nav_Menu_Toggle_Sanitizer extends WP_UnitTestCase {
 
@@ -22,9 +23,9 @@ public function data_converter() {
 		$container_id = 'nav-menu-container';
 		$toggle_id    = 'nav-menu-toggle';
 
-		$head      = sprintf( '<head><meta http-equiv="content-type" content="text/html; charset=%s"></head>', get_bloginfo( 'charset' ) );
 		$container = '<div id="' . esc_attr( $container_id ) . '" class="nav-menu-wrapper"></div>';
 		$toggle    = '<button id="' . esc_attr( $toggle_id ) . '">Toggle</button>';
+		$head      = '<head><meta charset="utf-8"></head>';
 
 		$amp_state               = '<amp-state id="navMenuToggledOn"><script type="application/json">false</script></amp-state>';
 		$amp_get_container_attrs = function( $class = '', $toggle_class = 'toggled-on' ) {
@@ -39,7 +40,7 @@ public function data_converter() {
 
 		return [
 			'container_before_toggle' => [
-				'<html>' . $head . '<body>' . $container . $toggle . '</body></html>',
+				'<html><body>' . $container . $toggle . '</body></html>',
 				'<html>' . $head . '<body>' . $amp_state . str_replace( '></div>', $amp_get_container_attrs( 'nav-menu-wrapper' ) . '></div>', $container ) . str_replace( '>Toggle', $amp_get_toggle_attrs() . '>Toggle', $toggle ) . '</body></html>',
 				[
 					'nav_container_id'           => $container_id,
@@ -49,7 +50,7 @@ public function data_converter() {
 				],
 			],
 			'toggle_before_container' => [
-				'<html>' . $head . '<body>' . $toggle . $container . '</body></html>',
+				'<html><body>' . $toggle . $container . '</body></html>',
 				'<html>' . $head . '<body>' . str_replace( '>Toggle', $amp_get_toggle_attrs() . '>Toggle', $toggle ) . $amp_state . str_replace( '></div>', $amp_get_container_attrs( 'nav-menu-wrapper' ) . '></div>', $container ) . '</body></html>',
 				[
 					'nav_container_id'           => $container_id,
@@ -59,7 +60,7 @@ public function data_converter() {
 				],
 			],
 			'container_is_body'       => [
-				'<html>' . $head . '<body>' . $container . $toggle . '</body></html>',
+				'<html><body>' . $container . $toggle . '</body></html>',
 				'<html>' . $head . '<body' . $amp_get_container_attrs( '', 'nav-menu-toggled-on' ) . '>' . $amp_state . $container . str_replace( '>Toggle', $amp_get_toggle_attrs( '', '' ) . '>Toggle', $toggle ) . '</body></html>',
 				[
 					'nav_container_xpath'        => '//body',
@@ -68,7 +69,7 @@ public function data_converter() {
 				],
 			],
 			'container_is_html'       => [
-				'<html>' . $head . '<body>' . $container . $toggle . '</body></html>',
+				'<html><body>' . $container . $toggle . '</body></html>',
 				'<html' . $amp_get_container_attrs( '', 'nav-menu-toggled-on' ) . '>' . $head . '<body>' . $amp_state . $container . str_replace( '>Toggle', $amp_get_toggle_attrs( '', '' ) . '>Toggle', $toggle ) . '</body></html>',
 				[
 					'nav_container_xpath'        => '//html',
@@ -77,7 +78,7 @@ public function data_converter() {
 				],
 			],
 			'no_container_provided'   => [
-				'<html>' . $head . '<body>' . $container . $toggle . '</body></html>',
+				'<html><body>' . $container . $toggle . '</body></html>',
 				'<html>' . $head . '<body>' . $container . '</body></html>',
 				[
 					'menu_button_id'             => $toggle_id,
@@ -85,7 +86,7 @@ public function data_converter() {
 				],
 			],
 			'no_arguments_provided'   => [
-				'<html>' . $head . '<body>' . $container . $toggle . '</body></html>',
+				'<html><body>' . $container . $toggle . '</body></html>',
 				'<html>' . $head . '<body>' . $container . $toggle . '</body></html>',
 				[],
 			],
@@ -105,12 +106,12 @@ public function data_converter() {
 	 * @covers AMP_Nav_Menu_Toggle_Sanitizer::get_menu_button()
 	 */
 	public function test_converter( $source, $expected, $args = [] ) {
-		$dom       = AMP_DOM_Utils::get_dom( $source );
+		$dom       = Document::from_html( $source );
 		$sanitizer = new AMP_Nav_Menu_Toggle_Sanitizer( $dom, $args );
 
 		$sanitizer->sanitize();
 
-		$content = AMP_DOM_Utils::get_content_from_dom_node( $dom, $dom->documentElement );
+		$content = $dom->saveHTML( $dom->documentElement );
 
 		$this->assertEquals( $expected, $content );
 	}
diff --git a/tests/php/test-class-amp-theme-support.php b/tests/php/test-class-amp-theme-support.php
index a073c9dfda8..2efcf8a44e0 100644
--- a/tests/php/test-class-amp-theme-support.php
+++ b/tests/php/test-class-amp-theme-support.php
@@ -6,6 +6,7 @@
  * @since 0.7
  */
 
+use Amp\AmpWP\Dom\Document;
 use org\bovigo\vfs;
 use Amp\AmpWP\Tests\PrivateAccess;
 
@@ -555,7 +556,7 @@ public function test_validate_non_amp_theme() {
 		<!DOCTYPE html>
 		<html lang="en-US" class="no-js">
 			<head>
-				<meta charset="UTF-8">
+				<meta charset="utf-8">
 				<meta name="viewport" content="maximum-scale=1.0">
 				<?php wp_head(); ?>
 			</head>
@@ -568,11 +569,11 @@ public function test_validate_non_amp_theme() {
 		$original_html  = trim( ob_get_clean() );
 		$sanitized_html = AMP_Theme_Support::prepare_response( $original_html );
 
-		// Invalid viewport meta tag is not present.
+		// Insufficient viewport tag was not left in.
 		$this->assertNotContains( '<meta name="viewport" content="maximum-scale=1.0">', $sanitized_html );
 
-		// Correct viewport meta tag was added.
-		$this->assertContains( '<meta name="viewport" content="width=device-width">', $sanitized_html );
+		// Viewport tag was modified to include all requirements.
+		$this->assertContains( '<meta name="viewport" content="width=device-width,maximum-scale=1.0">', $sanitized_html );
 
 		// MathML script was added.
 		$this->assertContains( '<script type="text/javascript" src="https://cdn.ampproject.org/v0/amp-mathml-0.1.js" async custom-element="amp-mathml"></script>', $sanitized_html ); // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedScript
@@ -1567,7 +1568,7 @@ public function get_schema_script_data() {
 	 */
 	public function test_ensure_required_markup_schemaorg( $script, $expected ) {
 		$page = '<html><head><script type="application/ld+json">%s</script></head><body>Test</body></html>';
-		$dom  = new DOMDocument();
+		$dom  = new Document();
 		$dom->loadHTML( sprintf( $page, $script ) );
 		AMP_Theme_Support::ensure_required_markup( $dom );
 		$this->assertEquals( $expected, substr_count( $dom->saveHTML(), 'schema.org' ) );
@@ -1599,10 +1600,9 @@ public function test_scripts_get_moved_to_head() {
 		$html = ob_get_clean();
 		$html = AMP_Theme_Support::prepare_response( $html );
 
-		$dom   = AMP_DOM_Utils::get_dom( $html );
-		$xpath = new DOMXPath( $dom );
+		$dom = Document::from_html( $html );
 
-		$scripts = $xpath->query( '//script[ not( @type ) or @type = "text/javascript" ]' );
+		$scripts = $dom->xpath->query( '//script[ not( @type ) or @type = "text/javascript" ]' );
 		$this->assertSame( 3, $scripts->length );
 		foreach ( $scripts as $script ) {
 			$this->assertSame( 'head', $script->parentNode->nodeName );
@@ -1659,12 +1659,11 @@ public function test_unneeded_scripts_get_removed() {
 		$html = ob_get_clean();
 		$html = AMP_Theme_Support::prepare_response( $html );
 
-		$dom   = AMP_DOM_Utils::get_dom( $html );
-		$xpath = new DOMXPath( $dom );
+		$dom = Document::from_html( $html );
 
 		/** @var DOMElement $script Script. */
 		$actual_script_srcs = [];
-		foreach ( $xpath->query( '//script[ not( @type ) or @type = "text/javascript" ]' ) as $script ) {
+		foreach ( $dom->xpath->query( '//script[ not( @type ) or @type = "text/javascript" ]' ) as $script ) {
 			$actual_script_srcs[] = $script->getAttribute( 'src' );
 		}
 
@@ -1712,8 +1711,7 @@ public function test_duplicate_scripts_are_removed() {
 		$html = ob_get_clean();
 		$html = AMP_Theme_Support::prepare_response( $html );
 
-		$dom   = AMP_DOM_Utils::get_dom( $html );
-		$xpath = new DOMXPath( $dom );
+		$dom = Document::from_html( $html );
 
 		$script_srcs = [];
 		/**
@@ -1721,7 +1719,7 @@ public function test_duplicate_scripts_are_removed() {
 		 *
 		 * @var DOMElement $script
 		 */
-		$scripts = $xpath->query( '//script[ @src ]' );
+		$scripts = $dom->xpath->query( '//script[ @src ]' );
 		foreach ( $scripts as $script ) {
 			$script_srcs[] = $script->getAttribute( 'src' );
 		}
@@ -1935,7 +1933,7 @@ static function ( $url ) {
 
 		$ordered_contains = [
 			'<html amp="">',
-			'<meta charset="' . get_bloginfo( 'charset' ) . '">',
+			'<meta charset="' . strtolower( get_bloginfo( 'charset' ) ) . '">',
 			'<meta name="viewport" content="width=device-width">',
 			'<meta name="generator" content="AMP Plugin',
 			'<title>',
@@ -2117,56 +2115,6 @@ public function test_prepare_response_for_submitted_form() {
 		unset( $_SERVER['REQUEST_METHOD'] );
 	}
 
-	/**
-	 * Test combining amp-script-src meta tags.
-	 *
-	 * @covers \amp_generate_script_hash()
-	 */
-	public function test_prepare_response_for_amp_script() {
-		$script1 = 'document.body.textContent += "First!";';
-		$script2 = 'document.body.textContent += "Second!";';
-		$script3 = 'document.body.textContent += "Third!";';
-		$script4 = 'document.body.textContent += "Fourth! (And forbidden because no amp-script-src meta in head.)";';
-
-		$script1_hash = amp_generate_script_hash( $script1 );
-		$script2_hash = amp_generate_script_hash( $script2 );
-		$script3_hash = amp_generate_script_hash( $script3 );
-		$script4_hash = amp_generate_script_hash( $script4 );
-
-		ob_start();
-		?>
-		<html>
-			<head>
-				<meta name="amp-script-src" content="<?php echo esc_attr( $script1_hash ); ?>">
-				<meta name="amp-script-src" content="<?php echo esc_attr( $script2_hash ); ?>">
-				<style>
-					body { background: black; color:white }
-				</style>
-				<meta name="amp-script-src" content="<?php echo esc_attr( $script3_hash ); ?>">
-			</head>
-			<body>
-				<meta name="amp-script-src" content="<?php echo esc_attr( $script4_hash ); ?>">
-
-				<amp-script script="s1" layout="fixed-height" height="30"></amp-script><script type="text/plain" target="amp-script" id="s1"><?php echo $script1; // phpcs:ignore ?></script>
-				<amp-script script="s2" layout="fixed-height" height="30"></amp-script><script type="text/plain" target="amp-script" id="s2"><?php echo $script2; // phpcs:ignore ?></script>
-				<amp-script script="s3" layout="fixed-height" height="30"></amp-script><script type="text/plain" target="amp-script" id="s3"><?php echo $script3; // phpcs:ignore ?></script>
-				<amp-script script="s4" layout="fixed-height" height="30"></amp-script><script type="text/plain" target="amp-script" id="s4"><?php echo $script4; // phpcs:ignore ?></script>
-			</body>
-		</html>
-		<?php
-		$output = ob_get_clean();
-
-		$processed = AMP_Theme_Support::prepare_response( $output );
-		$dom       = AMP_DOM_Utils::get_dom( $processed );
-		$xpath     = new DOMXPath( $dom );
-
-		$meta_elements = $xpath->query( '/html/head/meta[ @name = "amp-script-src" ]' );
-		$this->assertSame( 1, $meta_elements->length );
-
-		$meta = $meta_elements->item( 0 );
-		$this->assertSame( "$script1_hash $script2_hash $script3_hash", $meta->getAttribute( 'content' ) );
-	}
-
 	/**
 	 * Test post-processor cache effectiveness in AMP_Theme_Support::prepare_response().
 	 */
@@ -2376,38 +2324,13 @@ public function test_prepare_response_varying_html() {
 		$input  = '<html><head></head>Hello</html>';
 		$output = AMP_Theme_Support::prepare_response( $input );
 		$this->assertContains( '<html amp', $output );
-		$this->assertContains( '<meta charset="UTF-8">', $output );
+		$this->assertContains( '<meta charset="utf-8">', $output );
 
 		// HTML with doctype, comments, and whitespace before head.
 		$input  = "   <!--\nHello world!\n-->\n\n<!DOCTYPE html>  <html\n\n>\n<head profile='http://www.acme.com/profiles/core'></head><body>Hello</body></html>";
 		$output = AMP_Theme_Support::prepare_response( $input );
 		$this->assertContains( '<html amp', $output );
-		$this->assertContains( '<meta charset="UTF-8">', $output );
-	}
-
-	/**
-	 * Test prepare_response to inject html[amp] attribute and ensure HTML5 doctype.
-	 *
-	 * @covers AMP_Theme_Support::prepare_response()
-	 */
-	public function test_prepare_response_to_add_html5_doctype_and_amp_attribute() {
-		wp_scripts();
-		wp();
-		add_filter( 'amp_validation_error_sanitized', '__return_true' );
-		add_theme_support( AMP_Theme_Support::SLUG );
-		AMP_Theme_Support::init();
-		AMP_Theme_Support::finish_init();
-		ob_start();
-		?>
-		<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
-		<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><?php wp_head(); ?></head><body><?php wp_footer(); ?></body></html>
-		<?php
-		$original_html  = trim( ob_get_clean() );
-		$sanitized_html = AMP_Theme_Support::prepare_response( $original_html );
-
-		$this->assertStringStartsWith( '<!DOCTYPE html>', $sanitized_html );
-		$this->assertContains( '<html amp', $sanitized_html );
-		$this->assertContains( '<meta charset="utf-8">', $sanitized_html );
+		$this->assertContains( '<meta charset="utf-8">', $output );
 	}
 
 	/**
diff --git a/tests/php/test-dom-element-list.php b/tests/php/test-dom-element-list.php
index c555416fb83..38213b669e9 100644
--- a/tests/php/test-dom-element-list.php
+++ b/tests/php/test-dom-element-list.php
@@ -1,18 +1,18 @@
 <?php
 /**
- * Tests for DOMElementList, CaptionedSlide, and Image classes.
+ * Tests for ElementList and CaptionedSlide classes.
  *
  * @package AMP
  */
 
-use Amp\AmpWP\Component\DOMElementList;
-use Amp\AmpWP\Component\Image;
+use Amp\AmpWP\Dom\Document;
+use Amp\AmpWP\Dom\ElementList;
 use Amp\AmpWP\Component\CaptionedSlide;
 
 /**
  * Tests for AMP carousel and slide classes.
  *
- * @covers Amp\AmpWP\Component\DOMElementList, Amp\AmpWP\Component\CaptionedSlide, Amp\AmpWP\Component\Image
+ * @covers Amp\AmpWP\Dom\ElementList, Amp\AmpWP\Component\CaptionedSlide
  */
 class Test_DOM_Element_List extends \WP_UnitTestCase {
 
@@ -22,7 +22,7 @@ class Test_DOM_Element_List extends \WP_UnitTestCase {
 	 * @return array[] An associative array, including the images and the expected count.
 	 */
 	public function get_dom_element_list_data() {
-		$dom = new DOMDocument();
+		$dom = new Document();
 		return [
 			'no_image' => [
 				[],
@@ -48,14 +48,14 @@ public function get_dom_element_list_data() {
 	 * Test adding images and counting them.
 	 *
 	 * @dataProvider get_dom_element_list_data
-	 * @covers \Amp\AmpWP\Component\DOMElementList::add()
-	 * @covers \Amp\AmpWP\Component\DOMElementList::count()
+	 * @covers Amp\AmpWP\Dom\ElementList::add()
+	 * @covers Amp\AmpWP\Dom\ElementList::count()
 	 *
 	 * @param DOMElement[] $images         The images to add.
 	 * @param string       $expected_count The expected count after adding the images.
 	 */
 	public function test_dom_element_list_add( $images, $expected_count ) {
-		$dom_element_list = new DOMElementList();
+		$dom_element_list = new ElementList();
 		foreach ( $images as $image ) {
 			$dom_element_list = $dom_element_list->add( $image, '' );
 		}
@@ -67,14 +67,14 @@ public function test_dom_element_list_add( $images, $expected_count ) {
 	 * Test the iteration of the images.
 	 *
 	 * @dataProvider get_dom_element_list_data
-	 * @covers \Amp\AmpWP\Component\DOMElementList::add()
-	 * @covers \Amp\AmpWP\Component\DOMElementList::getIterator()
+	 * @covers Amp\AmpWP\Dom\ElementList::add()
+	 * @covers Amp\AmpWP\Dom\ElementList::getIterator()
 	 *
 	 * @param DOMElement[] $images         The images to add.
 	 * @param string       $expected_count The expected count after adding the images.
 	 */
 	public function test_dom_element_list_get_iterator( $images, $expected_count ) {
-		$dom_element_list = new DOMElementList();
+		$dom_element_list = new ElementList();
 		foreach ( $images as $image ) {
 			$dom_element_list = $dom_element_list->add( $image, '' );
 		}
@@ -94,7 +94,7 @@ public function test_dom_element_list_get_iterator( $images, $expected_count ) {
 	 * @covers \Amp\AmpWP\Component\CaptionedSlide::get_caption()
 	 */
 	public function test_get_caption() {
-		$image_node      = AMP_DOM_Utils::create_node( new DOMDocument(), 'amp-img', [] );
+		$image_node      = AMP_DOM_Utils::create_node( new Document(), 'amp-img', [] );
 		$caption         = 'This is a caption';
 		$captioned_image = new CaptionedSlide( $image_node, $caption );
 		$this->assertEquals( $caption, $captioned_image->get_caption() );
@@ -103,10 +103,10 @@ public function test_get_caption() {
 	/**
 	 * Test get_slide_node.
 	 *
-	 * @covers \Amp\AmpWP\Component\Image::get_slide_node()
+	 * @covers \Amp\AmpWP\Component\CaptionedSlide::get_slide_node()
 	 */
 	public function test_get_slide_node() {
-		$image_node = AMP_DOM_Utils::create_node( new DOMDocument(), 'amp-img', [] );
+		$image_node = AMP_DOM_Utils::create_node( new Document(), 'amp-img', [] );
 		$amp_image  = new CaptionedSlide( $image_node, '' );
 		$this->assertEquals( $image_node, $amp_image->get_slide_node() );
 	}
diff --git a/tests/php/test-tag-and-attribute-sanitizer.php b/tests/php/test-tag-and-attribute-sanitizer.php
index 163fcd3db27..b8069f5ecf9 100644
--- a/tests/php/test-tag-and-attribute-sanitizer.php
+++ b/tests/php/test-tag-and-attribute-sanitizer.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Test AMP_Tag_And_Attribute_Sanitizer
  *
@@ -2113,9 +2115,7 @@ public function get_html_data() {
 			],
 			'bad_meta_charset'                        => [
 				'<html amp><head><meta charset="latin-1"><title>Mojibake?</title></head><body></body></html>',
-				'<html amp><head><meta><title>Mojibake?</title></head><body></body></html>', // Note the charset attribute is removed because it violates the attribute spec, but the entire element is not removed because charset is not mandatory.
-				[],
-				[ AMP_Tag_And_Attribute_Sanitizer::DISALLOWED_ATTR ], // @todo Should actually be invalid_mandatory_attribute?
+				'<html amp><head><meta charset="utf-8"><title>Mojibake?</title></head><body></body></html>', // Note the charset attribute is removed because it violates the attribute spec, but the entire element is not removed because charset is not mandatory.
 			],
 			'bad_meta_viewport'                       => [
 				'<html amp><head><meta charset="utf-8"><meta name="viewport" content="maximum-scale=1.0"></head><body></body></html>',
@@ -2148,7 +2148,7 @@ public function get_html_data() {
 			],
 			'head_with_duplicate_charset'             => [
 				'<html amp><head><meta charset="UTF-8"><meta charset="utf-8"><body><p>Content</p></body></html>',
-				'<html amp><head><meta charset="UTF-8"></head><body><p>Content</p></body></html>',
+				'<html amp><head><meta charset="utf-8"></head><body><p>Content</p></body></html>',
 				[],
 				[ AMP_Tag_And_Attribute_Sanitizer::DUPLICATE_UNIQUE_TAG ],
 			],
@@ -2312,7 +2312,7 @@ public function get_html_data() {
 	 */
 	public function test_sanitize( $source, $expected = null, $expected_scripts = [], $expected_errors = [] ) {
 		$expected      = isset( $expected ) ? $expected : $source;
-		$dom           = AMP_DOM_Utils::get_dom( $source );
+		$dom           = Document::from_html( $source );
 		$actual_errors = [];
 		$sanitizer     = new AMP_Tag_And_Attribute_Sanitizer(
 			$dom,
@@ -2325,7 +2325,7 @@ public function test_sanitize( $source, $expected = null, $expected_scripts = []
 			]
 		);
 		$sanitizer->sanitize();
-		$content = AMP_DOM_Utils::get_content_from_dom_node( $dom, $dom->documentElement );
+		$content = $dom->saveHTML( $dom->documentElement );
 		$this->assertEqualMarkup( $expected, $content );
 
 		$this->assertEqualSets( $expected_scripts, array_keys( $sanitizer->get_scripts() ) );
@@ -2355,7 +2355,7 @@ public function test_sanitize_body_only() {
 		$source   = '<b>Hello</b><script>document.write("hi");</script><amp-sidebar></amp-sidebar>';
 		$expected = '<b>Hello</b><amp-sidebar></amp-sidebar>';
 
-		$dom           = AMP_DOM_Utils::get_dom( $source );
+		$dom           = Document::from_html( $source );
 		$actual_errors = [];
 		$sanitizer     = new AMP_Tag_And_Attribute_Sanitizer(
 			$dom,
@@ -2394,7 +2394,7 @@ public function test_is_missing_mandatory_attribute() {
 			],
 			'noloading'   => [],
 		];
-		$dom  = new DOMDocument();
+		$dom  = new Document();
 		$node = new DOMElement( 'amp-gist' );
 		$dom->appendChild( $node );
 		$sanitizer = new AMP_Tag_And_Attribute_Sanitizer( $dom );
diff --git a/tests/php/validation/test-class-amp-validation-manager.php b/tests/php/validation/test-class-amp-validation-manager.php
index 7874e2a0352..c272ba0311a 100644
--- a/tests/php/validation/test-class-amp-validation-manager.php
+++ b/tests/php/validation/test-class-amp-validation-manager.php
@@ -7,6 +7,8 @@
 
 // phpcs:disable Generic.Formatting.MultipleStatementAlignment.NotSameWarning
 
+use Amp\AmpWP\Dom\Document;
+
 /**
  * Tests for AMP_Validation_Manager class.
  *
@@ -89,8 +91,9 @@ class Test_AMP_Validation_Manager extends WP_UnitTestCase {
 	public function setUp() {
 		unset( $GLOBALS['wp_scripts'], $GLOBALS['wp_styles'] );
 		parent::setUp();
-		$dom_document = new DOMDocument( '1.0', 'utf-8' );
+		$dom_document = new Document( '1.0', 'utf-8' );
 		$this->node   = $dom_document->createElement( self::TAG_NAME );
+		$dom_document->appendChild( $this->node );
 		AMP_Validation_Manager::reset_validation_results();
 		$this->original_wp_registered_widgets = $GLOBALS['wp_registered_widgets'];
 
@@ -814,8 +817,7 @@ public function test_source_comments() {
 		 * @var DOMComment[] $comments
 		 */
 		$comments = [];
-		$xpath    = new DOMXPath( $dom );
-		foreach ( $xpath->query( '//comment()' ) as $comment ) {
+		foreach ( $dom->xpath->query( '//comment()' ) as $comment ) {
 			$comments[] = $comment;
 		}
 		$this->assertCount( 4, $comments );
@@ -1607,7 +1609,7 @@ public function test_finalize_validation() {
 		<?php
 		$html = ob_get_clean();
 
-		$dom = new DOMDocument();
+		$dom = new Document();
 		$dom->loadHTML( $html );
 
 		$this->assertInstanceOf( 'DOMElement', $dom->getElementById( 'wp-admin-bar-amp' ) );