diff --git a/includes/class-amp-theme-support.php b/includes/class-amp-theme-support.php
index 52eca9d8b74..f7401cd9700 100644
--- a/includes/class-amp-theme-support.php
+++ b/includes/class-amp-theme-support.php
@@ -193,8 +193,6 @@ public static function register_hooks() {
 		 */
 		add_action( 'template_redirect', array( __CLASS__, 'start_output_buffering' ), 0 );
 
-		add_filter( 'the_content', array( __CLASS__, 'filter_the_content' ), PHP_INT_MAX );
-
 		// @todo Add character conversion.
 	}
 
@@ -442,32 +440,12 @@ public static function get_amp_custom_styles() {
 		return $css;
 	}
 
-	/**
-	 * Filter the content to be valid AMP.
-	 *
-	 * @param string $content Content.
-	 * @return string Amplified content.
-	 */
-	public static function filter_the_content( $content ) {
-		$args = array(
-			'content_max_width' => ! empty( $content_width ) ? $content_width : AMP_Post_Template::CONTENT_MAX_WIDTH, // Back-compat.
-		);
-
-		list( $sanitized_content, $scripts, $styles ) = AMP_Content_Sanitizer::sanitize( $content, self::$sanitizer_classes, $args );
-
-		self::$amp_scripts = array_merge( self::$amp_scripts, $scripts );
-		self::$amp_styles  = array_merge( self::$amp_styles, $styles );
-
-		return $sanitized_content;
-	}
-
 	/**
 	 * Determine required AMP scripts.
 	 *
-	 * @param string $html Output HTML.
 	 * @return string Scripts to inject into the HEAD.
 	 */
-	public static function get_amp_component_scripts( $html ) {
+	public static function get_amp_component_scripts() {
 		$amp_scripts = self::$amp_scripts;
 
 		foreach ( self::$embed_handlers as $embed_handler ) {
@@ -512,15 +490,41 @@ public static function start_output_buffering() {
 	 * Finish output buffering.
 	 *
 	 * @todo Do this in shutdown instead of output buffering callback?
+	 * @global int $content_width
 	 * @param string $output Buffered output.
 	 * @return string Finalized output.
 	 */
 	public static function finish_output_buffering( $output ) {
+		global $content_width;
+
+		$dom  = AMP_DOM_Utils::get_dom( $output );
+		$args = array(
+			'content_max_width' => ! empty( $content_width ) ? $content_width : AMP_Post_Template::CONTENT_MAX_WIDTH, // Back-compat.
+		);
+
+		$assets = AMP_Content_Sanitizer::sanitize_document( $dom, self::$sanitizer_classes, $args );
+
+		self::$amp_scripts = array_merge( self::$amp_scripts, $assets['scripts'] );
+		self::$amp_styles  = array_merge( self::$amp_styles, $assets['styles'] );
+
+		/*
+		 * @todo The sanitize method needs to be updated to sanitize the entire HTML element and not just the BODY.
+		 * This will require updating mandatory_parent_blacklist in amphtml-update.py to include elements that appear in the HEAD.
+		 * This will ensure that the scripts and styles that plugins output via wp_head() will be sanitized as well. However,
+		 * since the the old paired mode is sending content from the *body* we'll need to be able to filter out the elements
+		 * from outside the body from being part of the whitelist sanitizer when it runs when theme support is not present,
+		 * as otherwise elements from the HEAD could get added to the BODY.
+		 */
+		$output = preg_replace(
+			'#(<body.*?>)(.+)(</body>)#si',
+			'$1' . AMP_DOM_Utils::get_content_from_dom( $dom ) . '$3',
+			$output
+		);
 
 		// Inject required scripts.
 		$output = preg_replace(
 			'#' . preg_quote( self::COMPONENT_SCRIPTS_PLACEHOLDER, '#' ) . '#',
-			self::get_amp_component_scripts( $output ),
+			self::get_amp_component_scripts(),
 			$output,
 			1
 		);
@@ -533,7 +537,6 @@ public static function finish_output_buffering( $output ) {
 			1
 		);
 
-		// @todo Add more validation checking and potentially the whitelist sanitizer.
 		return $output;
 	}
 }
diff --git a/includes/sanitizers/class-amp-rule-spec.php b/includes/sanitizers/class-amp-rule-spec.php
index 694b7dcf7df..6bec8aa167a 100644
--- a/includes/sanitizers/class-amp-rule-spec.php
+++ b/includes/sanitizers/class-amp-rule-spec.php
@@ -87,22 +87,18 @@ abstract class AMP_Rule_Spec {
 	 */
 	public static $additional_allowed_tags = array(
 
-		/**
-		 * An experimental tag with no protoascii
-		 */
+		// An experimental tag with no protoascii.
 		'amp-share-tracking' => array(
 			'attr_spec_list' => array(),
 			'tag_spec'       => array(),
 		),
 
-		/**
-		 *  Needed for some tags such as analytics
-		 */
+		// Needed for some tags such as analytics.
 		'script'             => array(
 			'attr_spec_list' => array(
 				'type' => array(
 					'mandatory'   => true,
-					'value_casei' => 'text/javascript',
+					'value_casei' => 'application/json',
 				),
 			),
 			'tag_spec'       => array(),
diff --git a/includes/templates/class-amp-content-sanitizer.php b/includes/templates/class-amp-content-sanitizer.php
index dcbd62097ef..ddf533091cd 100644
--- a/includes/templates/class-amp-content-sanitizer.php
+++ b/includes/templates/class-amp-content-sanitizer.php
@@ -7,23 +7,50 @@
 
 /**
  * Class AMP_Content_Sanitizer
+ *
+ * @since 0.4.1
  */
 class AMP_Content_Sanitizer {
 
 	/**
-	 * Sanitize.
+	 * Sanitize _content_.
+	 *
+	 * @since 0.4.1
 	 *
-	 * @param string   $content           Content.
+	 * @param string   $content HTML content string or DOM document.
 	 * @param string[] $sanitizer_classes Sanitizer classes.
 	 * @param array    $global_args       Global args.
-	 *
-	 * @return array
+	 * @return array Tuple containing sanitized HTML, scripts array, and styles array.
 	 */
 	public static function sanitize( $content, array $sanitizer_classes, $global_args = array() ) {
+		$dom = AMP_DOM_Utils::get_dom_from_content( $content );
+
+		$results = self::sanitize_document( $dom, $sanitizer_classes, $global_args );
+		return array(
+			AMP_DOM_Utils::get_content_from_dom( $dom ),
+			$results['scripts'],
+			$results['styles'],
+		);
+	}
+
+	/**
+	 * Sanitize document.
+	 *
+	 * @since 0.7
+	 *
+	 * @param DOMDocument $dom               HTML document.
+	 * @param string[]    $sanitizer_classes Sanitizer classes.
+	 * @param array       $global_args       Global args passed into .
+	 * @return array {
+	 *     Scripts and styles needed by sanitizers.
+	 *
+	 *     @type array $scripts Scripts.
+	 *     @type array $styles  Styles.
+	 * }
+	 */
+	public static function sanitize_document( &$dom, $sanitizer_classes, $global_args ) {
 		$scripts = array();
 		$styles  = array();
-		$dom     = AMP_DOM_Utils::get_dom_from_content( $content );
-
 		foreach ( $sanitizer_classes as $sanitizer_class => $args ) {
 			if ( ! class_exists( $sanitizer_class ) ) {
 				/* translators: %s is sanitizer class */
@@ -31,6 +58,11 @@ public static function sanitize( $content, array $sanitizer_classes, $global_arg
 				continue;
 			}
 
+			/**
+			 * Sanitizer.
+			 *
+			 * @type AMP_Base_Sanitizer $sanitizer
+			 */
 			$sanitizer = new $sanitizer_class( $dom, array_merge( $global_args, $args ) );
 
 			if ( ! is_subclass_of( $sanitizer, 'AMP_Base_Sanitizer' ) ) {
@@ -45,9 +77,7 @@ public static function sanitize( $content, array $sanitizer_classes, $global_arg
 			$styles  = array_merge( $styles, $sanitizer->get_styles() );
 		}
 
-		$sanitized_content = AMP_DOM_Utils::get_content_from_dom( $dom );
-
-		return array( $sanitized_content, $scripts, $styles );
+		return compact( 'scripts', 'styles' );
 	}
 }
 
diff --git a/includes/utils/class-amp-dom-utils.php b/includes/utils/class-amp-dom-utils.php
index d5f54bcbbf4..9492dd4cee5 100644
--- a/includes/utils/class-amp-dom-utils.php
+++ b/includes/utils/class-amp-dom-utils.php
@@ -13,17 +13,45 @@
 class AMP_DOM_Utils {
 
 	/**
-	 * Return a valid DOMDocument representing arbitrary HTML content passed as a parameter.
+	 * HTML elements that are self-closing.
 	 *
-	 * @see Reciprocal function get_content_from_dom()
+	 * Not all are valid AMP, but we include them for completeness.
 	 *
-	 * @since 0.2
+	 * @since 0.7
+	 * @link https://www.w3.org/TR/html5/syntax.html#serializing-html-fragments
+	 * @var array
+	 */
+	private static $self_closing_tags = array(
+		'area',
+		'base',
+		'basefont',
+		'bgsound',
+		'br',
+		'col',
+		'embed',
+		'frame',
+		'hr',
+		'img',
+		'input',
+		'keygen',
+		'link',
+		'meta',
+		'param',
+		'source',
+		'track',
+		'wbr',
+	);
+
+	/**
+	 * Return a valid DOMDocument representing HTML document passed as a parameter.
 	 *
-	 * @param string $content Valid HTML content to be represented by a DOMDocument.
+	 * @since 0.7
+	 *
+	 * @param string $document Valid HTML document to be represented by a DOMDocument.
 	 *
 	 * @return DOMDocument|false Returns DOMDocument, or false if conversion failed.
 	 */
-	public static function get_dom_from_content( $content ) {
+	public static function get_dom( $document ) {
 		$libxml_previous_state = libxml_use_internal_errors( true );
 
 		$dom = new DOMDocument();
@@ -34,13 +62,7 @@ public static function get_dom_from_content( $content ) {
 		 * We can later use this to extract our nodes.
 		 * Add charset so loadHTML does not have problems parsing it.
 		 */
-		$result = $dom->loadHTML(
-			sprintf(
-				'<html><head><meta http-equiv="content-type" content="text/html; charset=%s"></head><body>%s</body></html>',
-				get_bloginfo( 'charset' ),
-				$content
-			)
-		);
+		$result = $dom->loadHTML( $document );
 
 		libxml_clear_errors();
 		libxml_use_internal_errors( $libxml_previous_state );
@@ -52,6 +74,35 @@ public static function get_dom_from_content( $content ) {
 		return $dom;
 	}
 
+	/**
+	 * Return a valid DOMDocument 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.
+	 *
+	 * @return DOMDocument|false Returns DOMDocument, or false if conversion failed.
+	 */
+	public static function get_dom_from_content( $content ) {
+		/*
+		 * 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 );
+
+	}
+
 	/**
 	 * Return valid HTML content extracted from the DOMDocument passed as a parameter.
 	 *
@@ -67,6 +118,8 @@ public static function get_content_from_dom( $dom ) {
 
 		/**
 		 * We only want children of the body tag, since we have a subset of HTML.
+		 *
+		 * @todo We will want to get the full HTML eventually.
 		 */
 		$body = $dom->getElementsByTagName( 'body' )->item( 0 );
 
@@ -119,7 +172,7 @@ public static function get_content_from_dom_node( $dom, $node ) {
 		 * Cache this regex so we don't have to recreate it every call.
 		 */
 		if ( ! isset( $self_closing_tags_regex ) ) {
-			$self_closing_tags       = implode( '|', self::get_self_closing_tags() );
+			$self_closing_tags       = implode( '|', self::$self_closing_tags );
 			$self_closing_tags_regex = "#></({$self_closing_tags})>#i";
 		}
 
@@ -262,48 +315,6 @@ public static function recursive_force_closing_tags( $dom, $node = null ) {
 	 * @return bool Returns true if a valid self-closing tag, false if not.
 	 */
 	private static function is_self_closing_tag( $tag ) {
-		return in_array( $tag, self::get_self_closing_tags(), true );
-	}
-
-	/**
-	 * Returns array of self closing tags
-	 *
-	 * @since 0.6
-	 *
-	 * @return string[]
-	 */
-	private static function get_self_closing_tags() {
-		/*
-		 * As this function is called a lot the static var
-		 * prevents having to re-create the array every time.
-		 */
-		static $self_closing_tags;
-		if ( ! isset( $self_closing_tags ) ) {
-			/*
-			 * https://www.w3.org/TR/html5/syntax.html#serializing-html-fragments
-			 * Not all are valid AMP, but we include them for completeness.
-			 */
-			$self_closing_tags = array(
-				'area',
-				'base',
-				'basefont',
-				'bgsound',
-				'br',
-				'col',
-				'embed',
-				'frame',
-				'hr',
-				'img',
-				'input',
-				'keygen',
-				'link',
-				'meta',
-				'param',
-				'source',
-				'track',
-				'wbr',
-			);
-		}
-		return $self_closing_tags;
+		return in_array( strtolower( $tag ), self::$self_closing_tags, true );
 	}
 }
diff --git a/tests/test-class-amp-theme-support.php b/tests/test-class-amp-theme-support.php
index 5a10663bfb7..0b15e7f1165 100644
--- a/tests/test-class-amp-theme-support.php
+++ b/tests/test-class-amp-theme-support.php
@@ -67,4 +67,53 @@ public function test_is_paired_available() {
 		$this->assertTrue( is_search() );
 		$this->assertFalse( AMP_Theme_Support::is_paired_available() );
 	}
+
+	/**
+	 * Test finish_output_buffering.
+	 *
+	 * @covers AMP_Theme_Support::finish_output_buffering()
+	 */
+	public function test_finish_output_buffering() {
+		add_theme_support( 'amp' );
+		AMP_Theme_Support::init();
+		ob_start();
+		?>
+		<!DOCTYPE html>
+		<html amp <?php language_attributes(); ?>>
+			<head>
+				<?php wp_head(); ?>
+				<script data-head>document.write('TODO: This needs to be sanitized as well once.');</script>
+			</head>
+			<body>
+				<img width="100" height="100" src="https://example.com/test.png">
+				<audio width="400" height="300" src="https://example.com/audios/myaudio.mp3"></audio>
+				<amp-ad type="a9"
+					width="300"
+					height="250"
+					data-aax_size="300x250"
+					data-aax_pubname="test123"
+					data-aax_src="302"></amp-ad>
+				<?php wp_footer(); ?>
+			</body>
+		</html>
+		<?php
+		$original_html  = trim( ob_get_clean() );
+		$sanitized_html = AMP_Theme_Support::finish_output_buffering( $original_html );
+
+		$this->assertContains( '<meta charset="utf-8">', $sanitized_html );
+		$this->assertContains( '<meta name="viewport" content="width=device-width,minimum-scale=1">', $sanitized_html );
+		$this->assertContains( '<style amp-boilerplate>', $sanitized_html );
+		$this->assertContains( '<style amp-custom>', $sanitized_html );
+		$this->assertContains( '<script async src="https://cdn.ampproject.org/v0.js"', $sanitized_html ); // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedScript
+		$this->assertContains( '<meta name="generator" content="AMP Plugin', $sanitized_html );
+
+		$this->assertNotContains( '<img', $sanitized_html );
+		$this->assertContains( '<amp-img', $sanitized_html );
+
+		$this->assertNotContains( '<audio', $sanitized_html );
+		$this->assertContains( '<amp-audio', $sanitized_html );
+		$this->assertContains( '<script async custom-element="amp-audio"', $sanitized_html );
+
+		$this->assertContains( '<script async custom-element="amp-ad"', $sanitized_html );
+	}
 }