Add sanitizer to fixup issues with core themes

includes/class-amp-autoloader.php

@@ -78,6 +78,7 @@ class AMP_Autoloader {
 		'AMP_Style_Sanitizer'                         => 'includes/sanitizers/class-amp-style-sanitizer',
 		'AMP_Tag_And_Attribute_Sanitizer'             => 'includes/sanitizers/class-amp-tag-and-attribute-sanitizer',
 		'AMP_Video_Sanitizer'                         => 'includes/sanitizers/class-amp-video-sanitizer',
+		'AMP_Core_Theme_Sanitizer'                    => 'includes/sanitizers/class-amp-core-theme-sanitizer',
 		'AMP_Customizer_Design_Settings'              => 'includes/settings/class-amp-customizer-design-settings',
 		'AMP_Customizer_Settings'                     => 'includes/settings/class-amp-customizer-settings',
 		'AMP_Content'                                 => 'includes/templates/class-amp-content',

includes/class-amp-theme-support.php

@@ -214,6 +214,14 @@ public static function register_paired_hooks() {
 	 */
 	public static function add_hooks() {
 
+		add_filter( 'amp_content_sanitizers', function( $sanitizers ) {
+			$sanitizers['AMP_Core_Theme_Sanitizer'] = array(
+				'template'   => get_template(),
+				'stylesheet' => get_stylesheet(),
+			);
+			return $sanitizers;
+		} );
+
 		// Remove core actions which are invalid AMP.
 		remove_action( 'wp_head', 'wp_post_preview_js', 1 );
 		remove_action( 'wp_head', 'print_emoji_detection_script', 7 );

includes/sanitizers/class-amp-core-theme-sanitizer.php

@@ -0,0 +1,91 @@
+<?php
+/**
+ * Class AMP_Core_Theme_Sanitizer.
+ *
+ * @package AMP
+ * @since 1.0
+ */
+
+/**
+ * Class AMP_Core_Theme_Sanitizer
+ *
+ * Fixes up common issues in core themes and others.
+ *
+ * @since 1.0
+ */
+class AMP_Core_Theme_Sanitizer extends AMP_Base_Sanitizer {
+
+	/**
+	 * Config for features needed by themes.
+	 *
+	 * @var array
+	 */
+	public $theme_features = array(
+		'twentyseventeen' => array(
+			'force_svg_support'              => array(),
+			'force_fixed_background_support' => array(),
+			// @todo Header video probably needs special support in the plugin generally.
+			// @todo Header image is not styled properly. Needs layout=responsive and other things?
+			// @todo Dequeue scripts and replace with AMP functionality where possible.
+		),
+	);
+
+	/**
+	 * Fix up core themes to do things in the AMP way.
+	 *
+	 * @since 1.0
+	 */
+	public function sanitize() {
+		$theme_features = array();
+
+		// Find theme features for core theme.
+		$theme_candidates = wp_array_slice_assoc( $this->args, array( 'stylesheet', 'template' ) );
+		foreach ( $theme_candidates as $theme_candidate ) {
+			if ( isset( $this->theme_features[ $theme_candidate ] ) ) {
+				$theme_features = $this->theme_features[ $theme_candidate ];
+				break;
+			}
+		}
+
+		// Allow specific theme features to be requested even if the theme is not in core.
+		if ( isset( $this->args['theme_features'] ) ) {
+			$theme_features = array_merge( $this->args['theme_features'], $theme_features );
+		}
+
+		foreach ( $theme_features as $theme_feature => $feature_args ) {
+			if ( method_exists( $this, $theme_feature ) ) {
+				call_user_func( array( $this, $theme_feature ), $feature_args );
+			}
+		}
+	}
+
+	/**
+	 * Force SVG support, replacing no-svg class name with svg class name.
+	 *
+	 * @link https://github.com/WordPress/wordpress-develop/blob/1af1f65a21a1a697fb5f33027497f9e5ae638453/src/wp-content/themes/twentyseventeen/assets/js/global.js#L211-L213
+	 * @link https://caniuse.com/#feat=svg
+	 */
+	public function force_svg_support() {
+		$this->dom->documentElement->setAttribute(
+			'class',
+			preg_replace(
+				'/(^|\s)no-svg(\s|$)/',
+				' svg ',
+				$this->dom->documentElement->getAttribute( 'class' )
+			)
+		);
+	}
+
+	/**
+	 * Force support for fixed background-attachment.
+	 *
+	 * @link https://github.com/WordPress/wordpress-develop/blob/1af1f65a21a1a697fb5f33027497f9e5ae638453/src/wp-content/themes/twentyseventeen/assets/js/global.js#L215-L217
+	 * @link https://caniuse.com/#feat=background-attachment
+	 */
+	public function force_fixed_background_support() {
+		$this->dom->documentElement->setAttribute(
+			'class',
+			$this->dom->documentElement->getAttribute( 'class' ) . ' background-fixed'
+		);
+	}
+}