<?php
/**
 * Class AMP_Theme_Support
 *
 * @package AMP
 */

/**
 * Class AMP_Theme_Support
 *
 * Callbacks for adding AMP-related things when theme support is added.
 */
class AMP_Theme_Support {

	/**
	 * Theme support slug.
	 *
	 * @var string
	 */
	const SLUG = 'amp';

	/**
	 * Response cache group name.
	 *
	 * @var string
	 */
	const RESPONSE_CACHE_GROUP = 'amp-response';

	/**
	 * Post-processor cache effectiveness group name.
	 *
	 * @var string
	 */
	const POST_PROCESSOR_CACHE_EFFECTIVENESS_GROUP = 'post_processor_cache_effectiveness_group';

	/**
	 * Post-processor cache effectiveness key name.
	 *
	 * @var string
	 */
	const POST_PROCESSOR_CACHE_EFFECTIVENESS_KEY = 'post_processor_cache_effectiveness';

	/**
	 * Cache miss threshold for determining when to disable post-processor cache.
	 *
	 * @var int
	 */
	const CACHE_MISS_THRESHOLD = 20;

	/**
	 * Cache miss URL option name.
	 *
	 * @var string
	 */
	const CACHE_MISS_URL_OPTION = 'amp_cache_miss_url';

	/**
	 * Sanitizer classes.
	 *
	 * @var array
	 */
	protected static $sanitizer_classes = array();

	/**
	 * Embed handlers.
	 *
	 * @var AMP_Base_Embed_Handler[]
	 */
	protected static $embed_handlers = array();

	/**
	 * Template types.
	 *
	 * @var array
	 */
	protected static $template_types = array(
		'paged', // Deprecated.
		'index',
		'404',
		'archive',
		'author',
		'category',
		'tag',
		'taxonomy',
		'date',
		'home',
		'front_page',
		'page',
		'search',
		'single',
		'embed',
		'singular',
		'attachment',
	);

	/**
	 * Start time when init was called.
	 *
	 * @since 1.0
	 * @var float
	 */
	public static $init_start_time;

	/**
	 * Whether output buffering has started.
	 *
	 * @since 0.7
	 * @var bool
	 */
	protected static $is_output_buffering = false;

	/**
	 * Theme support options that were added via option.
	 *
	 * @since 1.0
	 * @var bool
	 */
	protected static $support_added_via_option = false;

	/**
	 * Initialize.
	 *
	 * @since 0.7
	 */
	public static function init() {
		self::read_theme_support();
		if ( ! current_theme_supports( self::SLUG ) ) {
			return;
		}

		self::$init_start_time = microtime( true );

		require_once AMP__DIR__ . '/includes/amp-post-template-actions.php';

		add_action( 'widgets_init', array( __CLASS__, 'register_widgets' ) );

		/*
		 * Note that wp action is use instead of template_redirect because some themes/plugins output
		 * the response at this action and then short-circuit with exit. So this is why the the preceding
		 * action to template_redirect--the wp action--is used instead.
		 */
		add_action( 'wp', array( __CLASS__, 'finish_init' ), PHP_INT_MAX );
	}

	/**
	 * Determine whether theme support was added via admin option.
	 *
	 * @since 1.0
	 * @see AMP_Theme_Support::read_theme_support()
	 *
	 * @return bool Support added via option.
	 */
	public static function is_support_added_via_option() {
		return self::$support_added_via_option;
	}

	/**
	 * Check theme support args or add theme support if option is set in the admin.
	 *
	 * The DB option is only considered if the theme does not already explicitly support AMP.
	 *
	 * @see AMP_Theme_Support::is_support_added_via_option()
	 * @see AMP_Post_Type_Support::add_post_type_support() For where post type support is added, since it is irrespective of theme support.
	 */
	public static function read_theme_support() {
		$theme_support_option = AMP_Options_Manager::get_option( 'theme_support' );
		if ( current_theme_supports( self::SLUG ) ) {
			$args = self::get_theme_support_args();

			// Validate theme support usage.
			$keys = array( 'template_dir', 'comments_live_list', 'paired', 'templates_supported', 'available_callback' );

			if ( count( array_diff( array_keys( $args ), $keys ) ) !== 0 ) {
				_doing_it_wrong( 'add_theme_support', esc_html( sprintf(  // phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_trigger_error
					/* translators: 1: comma-separated list of expected keys, 2: comma-separated list of actual keys */
					__( 'Expected AMP theme support to keys (%1$s) but saw (%2$s)', 'amp' ),
					join( ', ', $keys ),
					join( ', ', array_keys( $args ) )
				) ), '1.0' );
			}

			if ( isset( $args['available_callback'] ) ) {
				_doing_it_wrong( 'add_theme_support', esc_html__( 'The available_callback is deprecated when adding amp theme support in favor of declaratively setting the supported_templates.', 'amp' ), '1.0' );
			}
			self::$support_added_via_option = false;
		} elseif ( 'disabled' !== $theme_support_option ) {
			add_theme_support( self::SLUG, array(
				'paired' => ( 'paired' === $theme_support_option ),
			) );
			self::$support_added_via_option = true;
		} elseif ( AMP_Validation_Manager::is_theme_support_forced() ) {
			add_theme_support( self::SLUG );
		}
	}

	/**
	 * Get the theme support args.
	 *
	 * This avoids having to repeatedly call `get_theme_support()`, check the args, shift an item off the array, and so on.
	 *
	 * @since 1.0
	 *
	 * @return array|false Theme support args, or false if theme support is not present.
	 */
	public static function get_theme_support_args() {
		if ( ! current_theme_supports( self::SLUG ) ) {
			return false;
		}
		$support = get_theme_support( self::SLUG );
		if ( true === $support ) {
			return array(
				'paired' => false,
			);
		}
		if ( ! isset( $support[0] ) || ! is_array( $support[0] ) ) {
			return array();
		}
		return $support[0];
	}

	/**
	 * Finish initialization once query vars are set.
	 *
	 * @since 0.7
	 */
	public static function finish_init() {
		if ( ! is_amp_endpoint() ) {

			// Redirect to AMP-less variable if AMP is not available for this URL and yet the query var is present.
			if ( isset( $_GET[ amp_get_slug() ] ) ) { // WPCS: csrf ok.
				self::redirect_ampless_url();
			}

			amp_add_frontend_actions();
			return;
		}

		self::ensure_proper_amp_location();

		$theme_support = self::get_theme_support_args();
		if ( ! empty( $theme_support['template_dir'] ) ) {
			self::add_amp_template_filters();
		}

		self::add_hooks();
		self::$sanitizer_classes = amp_get_content_sanitizers();
		self::$sanitizer_classes = AMP_Validation_Manager::filter_sanitizer_args( self::$sanitizer_classes );
		self::$embed_handlers    = self::register_content_embed_handlers();
		self::$sanitizer_classes['AMP_Embed_Sanitizer']['embed_handlers'] = self::$embed_handlers;

		foreach ( self::$sanitizer_classes as $sanitizer_class => $args ) {
			if ( method_exists( $sanitizer_class, 'add_buffering_hooks' ) ) {
				call_user_func( array( $sanitizer_class, 'add_buffering_hooks' ), $args );
			}
		}
	}

	/**
	 * Ensure that the current AMP location is correct.
	 *
	 * @since 1.0
	 *
	 * @param bool $exit Whether to exit after redirecting.
	 * @return bool Whether redirection was done. Naturally this is irrelevant if $exit is true.
	 */
	public static function ensure_proper_amp_location( $exit = true ) {
		$has_query_var = false !== get_query_var( amp_get_slug(), false ); // May come from URL param or endpoint slug.
		$has_url_param = isset( $_GET[ amp_get_slug() ] ); // WPCS: CSRF OK.

		if ( amp_is_canonical() ) {
			/*
			 * When AMP native/canonical, then when there is an /amp/ endpoint or ?amp URL param,
			 * then a redirect needs to be done to the URL without any AMP indicator in the URL.
			 */
			if ( $has_query_var || $has_url_param ) {
				return self::redirect_ampless_url( $exit );
			}
		} else {
			/*
			 * When in AMP paired mode *with* theme support, then the proper AMP URL has the 'amp' URL param
			 * and not the /amp/ endpoint. The URL param is now the exclusive way to mark AMP in paired mode
			 * when amp theme support present. This is important for plugins to be able to reliably call
			 * is_amp_endpoint() before the parse_query action.
			 */
			if ( $has_query_var && ! $has_url_param ) {
				$old_url = amp_get_current_url();
				$new_url = add_query_arg( amp_get_slug(), '', amp_remove_endpoint( $old_url ) );
				if ( $old_url !== $new_url ) {
					wp_safe_redirect( $new_url, 302 );
					// @codeCoverageIgnoreStart
					if ( $exit ) {
						exit;
					}
					return true;
					// @codeCoverageIgnoreEnd
				}
			}
		}
		return false;
	}

	/**
	 * Redirect to non-AMP version of the current URL, such as because AMP is canonical or there are unaccepted validation errors.
	 *
	 * If the current URL is already AMP-less then do nothing.
	 *
	 * @since 0.7
	 * @since 1.0 Added $exit param.
	 * @since 1.0 Renamed from redirect_canonical_amp().
	 *
	 * @param bool $exit Whether to exit after redirecting.
	 * @return bool Whether redirection was done. Naturally this is irrelevant if $exit is true.
	 */
	public static function redirect_ampless_url( $exit = true ) {
		$current_url = amp_get_current_url();
		$ampless_url = amp_remove_endpoint( $current_url );
		if ( $ampless_url === $current_url ) {
			return false;
		}

		/*
		 * Temporary redirect because AMP URL may return when blocking validation errors
		 * occur or when a non-canonical AMP theme is used.
		 */
		wp_safe_redirect( $ampless_url, 302 );
		// @codeCoverageIgnoreStart
		if ( $exit ) {
			exit;
		}
		return true;
		// @codeCoverageIgnoreEnd
	}

	/**
	 * Determines whether paired mode is available.
	 *
	 * When 'amp' theme support has not been added or canonical mode is enabled, then this returns false.
	 *
	 * @since 0.7
	 *
	 * @see amp_is_canonical()
	 * @return bool Whether available.
	 */
	public static function is_paired_available() {
		if ( ! current_theme_supports( self::SLUG ) ) {
			return false;
		}

		if ( amp_is_canonical() ) {
			return false;
		}

		$availability = self::get_template_availability();
		return $availability['supported'];
	}

	/**
	 * Determine whether the user is in the Customizer preview iframe.
	 *
	 * @since 0.7
	 *
	 * @return bool Whether in Customizer preview iframe.
	 */
	public static function is_customize_preview_iframe() {
		global $wp_customize;
		return is_customize_preview() && $wp_customize->get_messenger_channel();
	}

	/**
	 * Register filters for loading AMP-specific templates.
	 */
	public static function add_amp_template_filters() {
		foreach ( self::$template_types as $template_type ) {
			add_filter( "{$template_type}_template_hierarchy", array( __CLASS__, 'filter_amp_template_hierarchy' ) );
		}
	}

	/**
	 * Determine template availability of AMP for the given query.
	 *
	 * This is not intended to return whether AMP is available for a _specific_ post. For that, use `post_supports_amp()`.
	 *
	 * @since 1.0
	 * @global WP_Query $wp_query
	 * @see post_supports_amp()
	 *
	 * @param WP_Query|WP_Post|null $query Query or queried post. If null then the global query will be used.
	 * @return array {
	 *     Template availability.
	 *
	 *     @type bool        $supported Whether the template is supported in AMP.
	 *     @type bool|null   $immutable Whether the supported status is known to be unchangeable.
	 *     @type string|null $template  The ID of the matched template (conditional), such as 'is_singular', or null if nothing was matched.
	 *     @type string[]    $errors    List of the errors or reasons for why the template is not available.
	 * }
	 */
	public static function get_template_availability( $query = null ) {
		global $wp_query;
		if ( ! $query ) {
			$query = $wp_query;
		} elseif ( $query instanceof WP_Post ) {
			$post  = $query;
			$query = new WP_Query();
			if ( 'page' === $post->post_type ) {
				$query->set( 'page_id', $post->ID );
			} else {
				$query->set( 'p', $post->ID );
			}
			$query->queried_object    = $post;
			$query->queried_object_id = $post->ID;
			$query->parse_query_vars();
		}

		$default_response = array(
			'errors'    => array(),
			'supported' => false,
			'immutable' => null,
			'template'  => null,
		);

		if ( ! ( $query instanceof WP_Query ) ) {
			_doing_it_wrong( __METHOD__, esc_html__( 'No WP_Query available.', 'amp' ), '1.0' );
			return array_merge(
				$default_response,
				array( 'errors' => array( 'no_query_available' ) )
			);
		}

		$theme_support_args = self::get_theme_support_args();
		if ( false === $theme_support_args ) {
			return array_merge(
				$default_response,
				array( 'errors' => array( 'no_theme_support' ) )
			);
		}

		// Support available_callback from 0.7, though it is deprecated.
		if ( isset( $theme_support_args['available_callback'] ) && is_callable( $theme_support_args['available_callback'] ) ) {
			/**
			 * Queried object.
			 *
			 * @var WP_Post $queried_object
			 */
			$queried_object = $query->get_queried_object();
			if ( ( is_singular() || $query->is_posts_page ) && ! post_supports_amp( $queried_object ) ) {
				return array_merge(
					$default_response,
					array(
						'errors'    => array( 'no-post-support' ),
						'supported' => false,
						'immutable' => true,
					)
				);
			}

			$response = array_merge(
				$default_response,
				array(
					'supported' => call_user_func( $theme_support_args['available_callback'] ),
					'immutable' => true,
				)
			);
			if ( ! $response['supported'] ) {
				$response['errors'][] = 'available_callback';
			}
			return $response;
		}

		$all_templates_supported_by_theme_support = false;
		if ( isset( $theme_support_args['templates_supported'] ) ) {
			$all_templates_supported_by_theme_support = 'all' === $theme_support_args['templates_supported'];
		}
		$all_templates_supported = (
			$all_templates_supported_by_theme_support || AMP_Options_Manager::get_option( 'all_templates_supported' )
		);

		// Make sure global $wp_query is set in case of conditionals that unfortunately look at global scope.
		$prev_query = $wp_query;
		$wp_query   = $query; // WPCS: override ok.

		$matching_templates    = array();
		$supportable_templates = self::get_supportable_templates();
		foreach ( $supportable_templates as $id => $supportable_template ) {
			if ( empty( $supportable_template['callback'] ) ) {
				$callback = $id;
			} else {
				$callback = $supportable_template['callback'];
			}

			// If the callback is a method on the query, then call the method on the query itself.
			if ( is_string( $callback ) && 'is_' === substr( $callback, 0, 3 ) && method_exists( $query, $callback ) ) {
				$is_match = call_user_func( array( $query, $callback ) );
			} elseif ( is_callable( $callback ) ) {
				$is_match = call_user_func( $callback, $query );
			} else {
				/* translators: %s: the supportable template ID. */
				_doing_it_wrong( __FUNCTION__, esc_html( sprintf( __( 'Supportable template "%s" does not have a callable callback.', 'amp' ), $id ) ), '1.0' );
				$is_match = false;
			}

			if ( $is_match ) {
				$matching_templates[ $id ] = array(
					'template'  => $id,
					'supported' => ! empty( $supportable_template['supported'] ),
					'immutable' => ! empty( $supportable_template['immutable'] ),
				);
			}
		}

		// Restore previous $wp_query (if any).
		$wp_query = $prev_query; // WPCS: override ok.

		// Make sure children override their parents.
		$matching_template_ids = array_keys( $matching_templates );
		foreach ( array_diff( array_keys( $supportable_templates ), $matching_template_ids ) as $template_id ) {
			unset( $supportable_templates[ $template_id ] );
		}
		foreach ( $matching_template_ids as $id ) {
			$has_children = false;
			foreach ( $supportable_templates as $other_id => $supportable_template ) {
				if ( $other_id === $id ) {
					continue;
				}
				if ( isset( $supportable_template['parent'] ) && $id === $supportable_template['parent'] ) {
					$has_children = true;
					break;
				}
			}

			// Delete all matching parent templates since the child will override them.
			if ( ! $has_children ) {
				$supportable_template = $supportable_templates[ $id ];
				while ( ! empty( $supportable_template['parent'] ) ) {
					$parent               = $supportable_template['parent'];
					$supportable_template = $supportable_templates[ $parent ];

					// Let the child supported status override the parent's supported status.
					unset( $matching_templates[ $parent ] );
				}
			}
		}

		// The is_home() condition is the default so discard it if there are other matching templates.
		if ( count( $matching_templates ) > 1 && isset( $matching_templates['is_home'] ) ) {
			unset( $matching_templates['is_home'] );
		}

		/*
		 * If there are more than one matching templates, then something is probably not right.
		 * Template conditions need to be set up properly to prevent this from happening.
		 */
		if ( count( $matching_templates ) > 1 ) {
			_doing_it_wrong( __METHOD__, esc_html__( 'Did not expect there to be more than one matching template. Did you filter amp_supportable_templates to not honor the template hierarchy?', 'amp' ), '1.0' );
		}

		$matching_template = array_shift( $matching_templates );

		// If there aren't any matching templates left that are supported, then we consider it to not be available.
		if ( ! $matching_template ) {
			if ( $all_templates_supported ) {
				return array_merge(
					$default_response,
					array(
						'supported' => true,
					)
				);
			} else {
				return array_merge(
					$default_response,
					array( 'errors' => array( 'no_matching_template' ) )
				);
			}
		}
		$matching_template = array_merge( $default_response, $matching_template );

		// If there aren't any matching templates left that are supported, then we consider it to not be available.
		if ( empty( $matching_template['supported'] ) ) {
			$matching_template['errors'][] = 'template_unsupported';
		}

		// For singular queries, post_supports_amp() is given the final say.
		if ( $query->is_singular() || $query->is_posts_page ) {
			/**
			 * Queried object.
			 *
			 * @var WP_Post $queried_object
			 */
			$queried_object = $query->get_queried_object();
			if ( $queried_object instanceof WP_Post ) {
				$support_errors = AMP_Post_Type_Support::get_support_errors( $queried_object );
				if ( ! empty( $support_errors ) ) {
					$matching_template['errors']    = array_merge( $matching_template['errors'], $support_errors );
					$matching_template['supported'] = false;
				}
			}
		}

		return $matching_template;
	}

	/**
	 * Get the templates which can be supported.
	 *
	 * @return array Supportable templates.
	 */
	public static function get_supportable_templates() {
		$templates = array(
			'is_singular' => array(
				'label'       => __( 'Singular', 'amp' ),
				'description' => __( 'Required for the above content types.', 'amp' ),
			),
		);
		if ( 'page' === get_option( 'show_on_front' ) ) {
			$templates['is_front_page'] = array(
				'label'  => __( 'Homepage', 'amp' ),
				'parent' => 'is_singular',
			);
			if ( AMP_Post_Meta_Box::DISABLED_STATUS === get_post_meta( get_option( 'page_on_front' ), AMP_Post_Meta_Box::STATUS_POST_META_KEY, true ) ) {
				/* translators: %s: the URL to the edit post screen. */
				$templates['is_front_page']['description'] = sprintf( __( 'Currently disabled at the <a href="%s" target="_blank">page level</a>.', 'amp' ), esc_url( get_edit_post_link( get_option( 'page_on_front' ) ) ) );
			}

			// In other words, same as is_posts_page, *but* it not is_singular.
			$templates['is_home'] = array(
				'label' => __( 'Blog', 'amp' ),
			);
			if ( AMP_Post_Meta_Box::DISABLED_STATUS === get_post_meta( get_option( 'page_for_posts' ), AMP_Post_Meta_Box::STATUS_POST_META_KEY, true ) ) {
				/* translators: %s: the URL to the edit post screen. */
				$templates['is_home']['description'] = sprintf( __( 'Currently disabled at the <a href="%s" target="_blank">page level</a>.', 'amp' ), esc_url( get_edit_post_link( get_option( 'page_for_posts' ) ) ) );
			}
		} else {
			$templates['is_home'] = array(
				'label' => __( 'Homepage', 'amp' ),
			);
		}

		$templates = array_merge(
			$templates,
			array(
				'is_archive' => array(
					'label' => __( 'Archives', 'amp' ),
				),
				'is_author'  => array(
					'label'  => __( 'Author', 'amp' ),
					'parent' => 'is_archive',
				),
				'is_date'    => array(
					'label'  => __( 'Date', 'amp' ),
					'parent' => 'is_archive',
				),
				'is_search'  => array(
					'label' => __( 'Search', 'amp' ),
				),
				'is_404'     => array(
					'label' => __( 'Not Found (404)', 'amp' ),
				),
			)
		);

		if ( taxonomy_exists( 'category' ) ) {
			$templates['is_category'] = array(
				'label'  => get_taxonomy( 'category' )->labels->name,
				'parent' => 'is_archive',
			);
		}
		if ( taxonomy_exists( 'post_tag' ) ) {
			$templates['is_tag'] = array(
				'label'  => get_taxonomy( 'post_tag' )->labels->name,
				'parent' => 'is_archive',
			);
		}

		$taxonomy_args = array(
			'_builtin'           => false,
			'publicly_queryable' => true,
		);
		foreach ( get_taxonomies( $taxonomy_args, 'objects' ) as $taxonomy ) {
			$templates[ sprintf( 'is_tax[%s]', $taxonomy->name ) ] = array(
				'label'    => $taxonomy->labels->name,
				'parent'   => 'is_archive',
				'callback' => function ( WP_Query $query ) use ( $taxonomy ) {
					return $query->is_tax( $taxonomy->name );
				},
			);
		}

		$post_type_args = array(
			'has_archive'        => true,
			'publicly_queryable' => true,
		);
		foreach ( get_post_types( $post_type_args, 'objects' ) as $post_type ) {
			$templates[ sprintf( 'is_post_type_archive[%s]', $post_type->name ) ] = array(
				'label'    => $post_type->labels->archives,
				'parent'   => 'is_archive',
				'callback' => function ( WP_Query $query ) use ( $post_type ) {
					return $query->is_post_type_archive( $post_type->name );
				},
			);
		}

		/**
		 * Filters list of supportable templates.
		 *
		 * A theme or plugin can force a given template to be supported or not by preemptively
		 * setting the 'supported' flag for a given template. Otherwise, if the flag is undefined
		 * then the user will be able to toggle it themselves in the admin. Each array item should
		 * have a key that corresponds to a template conditional function. If the key is such a
		 * function, then the key is used to evaluate whether the given template entry is a match.
		 * Otherwise, a supportable template item can include a callback value which is used instead.
		 * Each item needs a 'label' value. Additionally, if the supportable template is a subset of
		 * another condition (e.g. is_singular > is_single) then this relationship needs to be
		 * indicated via the 'parent' value.
		 *
		 * @since 1.0
		 *
		 * @param array $templates Supportable templates.
		 */
		$templates = apply_filters( 'amp_supportable_templates', $templates );

		$theme_support_args        = self::get_theme_support_args();
		$theme_supported_templates = array();
		if ( isset( $theme_support_args['templates_supported'] ) ) {
			$theme_supported_templates = $theme_support_args['templates_supported'];
		}

		$supported_templates = AMP_Options_Manager::get_option( 'supported_templates' );
		foreach ( $templates as $id => &$template ) {

			// Capture user-elected support from options. This allows us to preserve the original user selection through programmatic overrides.
			$template['user_supported'] = in_array( $id, $supported_templates, true );

			// Consider supported templates from theme support args.
			if ( ! isset( $template['supported'] ) ) {
				if ( 'all' === $theme_supported_templates ) {
					$template['supported'] = true;
				} elseif ( is_array( $theme_supported_templates ) && isset( $theme_supported_templates[ $id ] ) ) {
					$template['supported'] = $theme_supported_templates[ $id ];
				}
			}

			// Make supported state immutable if it was programmatically set.
			$template['immutable'] = isset( $template['supported'] );

			// Set supported state from user preference.
			if ( ! $template['immutable'] ) {
				$template['supported'] = AMP_Options_Manager::get_option( 'all_templates_supported' ) || $template['user_supported'];
			}
		}

		return $templates;
	}

	/**
	 * Register hooks.
	 */
	public static function add_hooks() {

		// Let the AMP plugin manage service worker streaming in the PWA plugin.
		remove_action( 'template_redirect', 'WP_Service_Worker_Navigation_Routing_Component::start_output_buffering_stream_fragment', PHP_INT_MAX );

		// 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 );
		remove_action( 'wp_print_styles', 'print_emoji_styles' );
		remove_action( 'wp_head', 'wp_oembed_add_host_js' );

		// @todo The wp_mediaelement_fallback() should still run to be injected inside of the audio/video generated by wp_audio_shortcode()/wp_video_shortcode() respectively.
		// Prevent MediaElement.js scripts/styles from being enqueued.
		add_filter( 'wp_video_shortcode_library', function() {
			return 'amp';
		} );
		add_filter( 'wp_audio_shortcode_library', function() {
			return 'amp';
		} );

		// Don't show loading indicator on custom logo since it makes most sense for larger images.
		add_filter( 'get_custom_logo', function( $html ) {
			return preg_replace( '/(?<=<img\s)/', ' noloading ', $html );
		}, 1 );

		/*
		 * "AMP HTML documents MUST contain the AMP boilerplate code (head > style[amp-boilerplate] and noscript > style[amp-boilerplate])
		 * in their head tag." {@link https://www.ampproject.org/docs/fundamentals/spec#required-markup AMP Required markup}
		 *
		 * After "Specify the <link> tag for your favicon.", then
		 * "Specify any custom styles by using the <style amp-custom> tag."
		 *
		 * Note that the boilerplate is added at the very end because:
		 * "Finally, specify the AMP boilerplate code. By putting the boilerplate code last, it prevents custom styles from accidentally
		 * overriding the boilerplate css rules." {@link https://docs.google.com/document/d/169XUxtSSEJb16NfkrCr9y5lqhUR7vxXEAsNxBzg07fM/edit AMP Hosting Guide}
		 *
		 * Other required markup is added in the ensure_required_markup method, including meta charset, meta viewport, and rel=canonical link.
		 */
		add_action( 'wp_head', function() {
			echo '<style amp-custom></style>';
		}, 0 );
		add_action( 'wp_head', function() {
			echo amp_get_boilerplate_code(); // WPCS: xss ok.
		}, PHP_INT_MAX );

		add_action( 'wp_head', 'amp_add_generator_metadata', 20 );
		add_action( 'wp_enqueue_scripts', array( __CLASS__, 'enqueue_assets' ) );
		add_action( 'wp_enqueue_scripts', array( __CLASS__, 'dequeue_customize_preview_scripts' ), 1000 );
		add_filter( 'customize_partial_render', array( __CLASS__, 'filter_customize_partial_render' ) );

		add_action( 'wp_footer', 'amp_print_analytics' );

		/*
		 * Disable admin bar because admin-bar.css (28K) and Dashicons (48K) alone
		 * combine to surpass the 50K limit imposed for the amp-custom style.
		 */
		if ( AMP_Options_Manager::get_option( 'disable_admin_bar' ) ) {
			add_filter( 'show_admin_bar', '__return_false', 100 );
		} else {
			add_action( 'admin_bar_init', array( __CLASS__, 'init_admin_bar' ) );
		}

		/*
		 * Start output buffering at very low priority for sake of plugins and themes that use template_redirect
		 * instead of template_include.
		 */
		$priority = defined( 'PHP_INT_MIN' ) ? PHP_INT_MIN : ~PHP_INT_MAX; // phpcs:ignore PHPCompatibility.PHP.NewConstants.php_int_minFound
		add_action( 'template_redirect', array( __CLASS__, 'start_output_buffering' ), $priority );

		// Commenting hooks.
		add_filter( 'wp_list_comments_args', array( __CLASS__, 'set_comments_walker' ), PHP_INT_MAX );
		add_filter( 'comment_form_defaults', array( __CLASS__, 'filter_comment_form_defaults' ) );
		add_filter( 'comment_reply_link', array( __CLASS__, 'filter_comment_reply_link' ), 10, 4 );
		add_filter( 'cancel_comment_reply_link', array( __CLASS__, 'filter_cancel_comment_reply_link' ), 10, 3 );
		add_action( 'comment_form', array( __CLASS__, 'amend_comment_form' ), 100 );
		remove_action( 'comment_form', 'wp_comment_form_unfiltered_html_nonce' );
		add_filter( 'wp_kses_allowed_html', array( __CLASS__, 'whitelist_layout_in_wp_kses_allowed_html' ), 10 );
		add_filter( 'get_header_image_tag', array( __CLASS__, 'amend_header_image_with_video_header' ), PHP_INT_MAX );
		add_action( 'wp_print_footer_scripts', function() {
			wp_dequeue_script( 'wp-custom-header' );
		}, 0 );
		add_action( 'wp_enqueue_scripts', function() {
			wp_dequeue_script( 'comment-reply' ); // Handled largely by AMP_Comments_Sanitizer and *reply* methods in this class.
		} );

		// @todo Add character conversion.
	}

	/**
	 * Register/override widgets.
	 *
	 * @global WP_Widget_Factory
	 * @return void
	 */
	public static function register_widgets() {
		global $wp_widget_factory;
		foreach ( $wp_widget_factory->widgets as $registered_widget ) {
			$registered_widget_class_name = get_class( $registered_widget );
			if ( ! preg_match( '/^WP_Widget_(.+)$/', $registered_widget_class_name, $matches ) ) {
				continue;
			}
			$amp_class_name = 'AMP_Widget_' . $matches[1];
			if ( ! class_exists( $amp_class_name ) || is_a( $amp_class_name, $registered_widget_class_name ) ) {
				continue;
			}

			unregister_widget( $registered_widget_class_name );
			register_widget( $amp_class_name );
		}
	}

	/**
	 * Register content embed handlers.
	 *
	 * This was copied from `AMP_Content::register_embed_handlers()` due to being a private method
	 * and due to `AMP_Content` not being well suited for use in AMP canonical.
	 *
	 * @see AMP_Content::register_embed_handlers()
	 * @global int $content_width
	 * @return AMP_Base_Embed_Handler[] Handlers.
	 */
	public static function register_content_embed_handlers() {
		global $content_width;

		$embed_handlers = array();
		foreach ( amp_get_content_embed_handlers() as $embed_handler_class => $args ) {

			/**
			 * Embed handler.
			 *
			 * @type AMP_Base_Embed_Handler $embed_handler
			 */
			$embed_handler = new $embed_handler_class( array_merge(
				array(
					'content_max_width' => ! empty( $content_width ) ? $content_width : AMP_Post_Template::CONTENT_MAX_WIDTH, // Back-compat.
				),
				$args
			) );

			if ( ! is_subclass_of( $embed_handler, 'AMP_Base_Embed_Handler' ) ) {
				/* translators: %s is embed handler */
				_doing_it_wrong( __METHOD__, esc_html( sprintf( __( 'Embed Handler (%s) must extend `AMP_Embed_Handler`', 'amp' ), $embed_handler_class ) ), '0.1' );
				continue;
			}

			$embed_handler->register_embed();
			$embed_handlers[] = $embed_handler;
		}

		return $embed_handlers;
	}

	/**
	 * Add the comments template placeholder marker
	 *
	 * @param array $args the args for the comments list..
	 * @return array Args to return.
	 */
	public static function set_comments_walker( $args ) {
		$amp_walker     = new AMP_Comment_Walker();
		$args['walker'] = $amp_walker;
		return $args;
	}

	/**
	 * Adds the form submit success and fail templates.
	 */
	public static function amend_comment_form() {
		?>
		<?php if ( is_singular() && ! amp_is_canonical() ) : ?>
			<input type="hidden" name="redirect_to" value="<?php echo esc_url( amp_get_permalink( get_the_ID() ) ); ?>">
		<?php endif; ?>

		<div submit-success>
			<template type="amp-mustache">
				<p>{{{message}}}</p>
			</template>
		</div>
		<div submit-error>
			<template type="amp-mustache">
				<p class="amp-comment-submit-error">{{{error}}}</p>
			</template>
		</div>
		<?php
	}

	/**
	 * Prepends template hierarchy with template_dir for AMP paired mode templates.
	 *
	 * @param array $templates Template hierarchy.
	 * @return array Templates.
	 */
	public static function filter_amp_template_hierarchy( $templates ) {
		$args = self::get_theme_support_args();
		if ( isset( $args['template_dir'] ) ) {
			$amp_templates = array();
			foreach ( $templates as $template ) {
				$amp_templates[] = $args['template_dir'] . '/' . $template; // Let template_dir have precedence.
				$amp_templates[] = $template;
			}
			$templates = $amp_templates;
		}
		return $templates;
	}

	/**
	 * Get canonical URL for current request.
	 *
	 * @see rel_canonical()
	 * @global WP $wp
	 * @global WP_Rewrite $wp_rewrite
	 * @link https://www.ampproject.org/docs/reference/spec#canon.
	 * @link https://core.trac.wordpress.org/ticket/18660
	 *
	 * @return string Canonical non-AMP URL.
	 */
	public static function get_current_canonical_url() {
		global $wp, $wp_rewrite;

		$url = null;
		if ( is_singular() ) {
			$url = wp_get_canonical_url();
		}

		// For non-singular queries, make use of the request URI and public query vars to determine canonical URL.
		if ( empty( $url ) ) {
			$added_query_vars = $wp->query_vars;
			if ( ! $wp_rewrite->permalink_structure || empty( $wp->request ) ) {
				$url = home_url( '/' );
			} else {
				$url = home_url( user_trailingslashit( $wp->request ) );
				parse_str( $wp->matched_query, $matched_query_vars );
				foreach ( $wp->query_vars as $key => $value ) {

					// Remove query vars that were matched in the rewrite rules for the request.
					if ( isset( $matched_query_vars[ $key ] ) ) {
						unset( $added_query_vars[ $key ] );
					}
				}
			}
		}

		if ( ! empty( $added_query_vars ) ) {
			$url = add_query_arg( $added_query_vars, $url );
		}

		return amp_remove_endpoint( $url );
	}

	/**
	 * Get the ID for the amp-state.
	 *
	 * @since 0.7
	 *
	 * @param int $post_id Post ID.
	 * @return string ID for amp-state.
	 */
	public static function get_comment_form_state_id( $post_id ) {
		return sprintf( 'commentform_post_%d', $post_id );
	}

	/**
	 * Filter comment form args to an element with [text] AMP binding wrap the title reply.
	 *
	 * @since 0.7
	 * @see comment_form()
	 *
	 * @param array $args Comment form args.
	 * @return array Filtered comment form args.
	 */
	public static function filter_comment_form_defaults( $args ) {
		$state_id = self::get_comment_form_state_id( get_the_ID() );

		$text_binding = sprintf(
			'%s.replyToName ? %s : %s',
			$state_id,
			str_replace(
				'%s',
				sprintf( '" + %s.replyToName + "', $state_id ),
				wp_json_encode( $args['title_reply_to'] )
			),
			wp_json_encode( $args['title_reply'] )
		);

		$args['title_reply_before'] .= sprintf(
			'<span [text]="%s">',
			esc_attr( $text_binding )
		);
		$args['cancel_reply_before'] = '</span>' . $args['cancel_reply_before'];
		return $args;
	}

	/**
	 * Modify the comment reply link for AMP.
	 *
	 * @since 0.7
	 * @see get_comment_reply_link()
	 *
	 * @param string     $link    The HTML markup for the comment reply link.
	 * @param array      $args    An array of arguments overriding the defaults.
	 * @param WP_Comment $comment The object of the comment being replied.
	 * @return string Comment reply link.
	 */
	public static function filter_comment_reply_link( $link, $args, $comment ) {

		// Continue to show default link to wp-login when user is not logged-in.
		if ( get_option( 'comment_registration' ) && ! is_user_logged_in() ) {
			return $args['before'] . $link . $args['after'];
		}

		$state_id  = self::get_comment_form_state_id( get_the_ID() );
		$tap_state = array(
			$state_id => array(
				'replyToName' => $comment->comment_author,
				'values'      => array(
					'comment_parent' => (string) $comment->comment_ID,
				),
			),
		);

		// @todo Figure out how to support add_below. Instead of moving the form, what about letting the form get a fixed position?
		$link = sprintf(
			'<a rel="nofollow" class="comment-reply-link" href="%s" on="%s" aria-label="%s">%s</a>',
			esc_attr( '#' . $args['respond_id'] ),
			esc_attr( sprintf( 'tap:AMP.setState( %s )', wp_json_encode( $tap_state ) ) ),
			esc_attr( sprintf( $args['reply_to_text'], $comment->comment_author ) ),
			$args['reply_text']
		);
		return $args['before'] . $link . $args['after'];
	}

	/**
	 * Filters the cancel comment reply link HTML.
	 *
	 * @since 0.7
	 * @see get_cancel_comment_reply_link()
	 *
	 * @param string $formatted_link The HTML-formatted cancel comment reply link.
	 * @param string $link           Cancel comment reply link URL.
	 * @param string $text           Cancel comment reply link text.
	 * @return string Cancel reply link.
	 */
	public static function filter_cancel_comment_reply_link( $formatted_link, $link, $text ) {
		unset( $formatted_link, $link );
		if ( empty( $text ) ) {
			$text = __( 'Click here to cancel reply.', 'default' );
		}

		$state_id  = self::get_comment_form_state_id( get_the_ID() );
		$tap_state = array(
			$state_id => array(
				'replyToName' => '',
				'values'      => array(
					'comment_parent' => '0',
				),
			),
		);

		$respond_id = 'respond'; // Hard-coded in comment_form() and default value in get_comment_reply_link().
		return sprintf(
			'<a id="cancel-comment-reply-link" href="%s" %s [hidden]="%s" on="%s">%s</a>',
			esc_url( remove_query_arg( 'replytocom' ) . '#' . $respond_id ),
			isset( $_GET['replytocom'] ) ? '' : ' hidden', // phpcs:ignore
			esc_attr( sprintf( '%s.values.comment_parent == "0"', self::get_comment_form_state_id( get_the_ID() ) ) ),
			esc_attr( sprintf( 'tap:AMP.setState( %s )', wp_json_encode( $tap_state ) ) ),
			esc_html( $text )
		);
	}

	/**
	 * Configure the admin bar for AMP.
	 *
	 * @since 1.0
	 */
	public static function init_admin_bar() {

		// Replace admin-bar.css in core with forked version which makes use of :focus-within among other change for AMP-compat.
		wp_styles()->registered['admin-bar']->src = amp_get_asset_url( 'css/admin-bar.css' );
		wp_styles()->registered['admin-bar']->ver = AMP__VERSION;

		// Remove script which is almost entirely made obsolete by :focus-inside in the forked admin-bar.css.
		wp_dequeue_script( 'admin-bar' );

		// Remove customize support script since not valid AMP.
		add_action( 'admin_bar_menu', function() {
			remove_action( 'wp_before_admin_bar_render', 'wp_customize_support_script' );
		}, 41 );

		// Emulate customize support script in PHP, to assume Customizer.
		add_filter( 'body_class', function( $body_classes ) {
			return array_merge(
				array_diff(
					$body_classes,
					array( 'no-customize-support' )
				),
				array( 'customize-support' )
			);
		} );
	}

	/**
	 * Ensure the markup exists as required by AMP and elements are in the optimal loading order.
	 *
	 * Ensure meta[charset], meta[name=viewport], and link[rel=canonical] exist, as the whitelist sanitizer
	 * may have removed an illegal meta[http-equiv] or meta[name=viewport]. For a singular post, core only outputs a
	 * canonical URL by default. Adds the preload links.
	 *
	 * @since 0.7
	 * @link https://www.ampproject.org/docs/reference/spec#required-markup
	 * @link https://docs.google.com/document/d/169XUxtSSEJb16NfkrCr9y5lqhUR7vxXEAsNxBzg07fM/edit#heading=h.2ha259c3ffos
	 * @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.
	 */
	public static function ensure_required_markup( DOMDocument $dom, $script_handles = array() ) {
		/**
		 * Elements.
		 *
		 * @var DOMElement $meta
		 * @var DOMElement $script
		 * @var DOMElement $link
		 */

		$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 );
		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() ) ) );
			$head->appendChild( $script );
		}

		// Ensure rel=canonical link.
		$links         = array();
		$link_elements = $head->getElementsByTagName( 'link' );
		$rel_canonical = null;
		foreach ( $link_elements as $link ) {
			if ( $link->hasAttribute( 'rel' ) ) {
				$links[ $link->getAttribute( 'rel' ) ][] = $link;
			}
		}
		if ( empty( $links['canonical'] ) ) {
			$rel_canonical = AMP_DOM_Utils::create_node( $dom, 'link', array(
				'rel'  => 'canonical',
				'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."
		 *
		 * "The first tag should be the meta charset tag, followed by any remaining meta tags."
		 *
		 * {@link https://docs.google.com/document/d/169XUxtSSEJb16NfkrCr9y5lqhUR7vxXEAsNxBzg07fM/edit#heading=h.2ha259c3ffos Optimize the AMP Runtime loading}
		 */
		$meta_charset  = null;
		$meta_viewport = null;
		$meta_elements = array();
		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;
			} else {
				$meta_elements[] = $meta;
			}
		}
		if ( ! $meta_charset ) {
			// Warning: This probably means the character encoding needs to be converted.
			$meta_charset = AMP_DOM_Utils::create_node( $dom, 'meta', array(
				'charset' => 'utf-8',
			) );
		} else {
			$head->removeChild( $meta_charset ); // So we can move it.
		}
		$head->insertBefore( $meta_charset, $head->firstChild );

		if ( ! $meta_viewport ) {
			$meta_viewport = AMP_DOM_Utils::create_node( $dom, 'meta', array(
				'name'    => 'viewport',
				'content' => 'width=device-width',
			) );
		} else {
			$head->removeChild( $meta_viewport ); // So we can move it.
		}
		$head->insertBefore( $meta_viewport, $meta_charset->nextSibling );

		$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;
		}

		$title = $head->getElementsByTagName( 'title' )->item( 0 );
		if ( $title ) {
			$title->parentNode->removeChild( $title ); // So we can move it.
			$head->insertBefore( $title, $previous_node->nextSibling );
			$previous_node = $title;
		}

		// @see https://github.com/ampproject/amphtml/blob/2fd30ca984bceac05905bd5b17f9e0010629d719/src/render-delaying-services.js#L39-L43 AMPHTML Render Delaying Services SERVICES definition.
		$render_delaying_extensions = array(
			'amp-experiment',
			'amp-dynamic-css-classes',
			'amp-story',
		);

		// Obtain the existing AMP scripts.
		$amp_scripts     = array();
		$ordered_scripts = array();
		$head_scripts    = array();
		$runtime_src     = wp_scripts()->registered['amp-runtime']->src;
		foreach ( $head->getElementsByTagName( 'script' ) as $script ) { // Note that prepare_response() already moved body scripts to head.
			$head_scripts[] = $script;
		}
		foreach ( $head_scripts as $script ) {
			$src = $script->getAttribute( 'src' );
			if ( ! $src || 'https://cdn.ampproject.org/' !== substr( $src, 0, 27 ) ) {
				continue;
			}
			if ( $runtime_src === $src ) {
				$amp_scripts['amp-runtime'] = $script;
			} elseif ( $script->hasAttribute( 'custom-element' ) ) {
				$amp_scripts[ $script->getAttribute( 'custom-element' ) ] = $script;
			} elseif ( $script->hasAttribute( 'custom-template' ) ) {
				$amp_scripts[ $script->getAttribute( 'custom-template' ) ] = $script;
			} else {
				continue;
			}
			$script->parentNode->removeChild( $script ); // So we can move it.
		}

		// Create scripts for any components discovered from output buffering.
		foreach ( array_diff( $script_handles, array_keys( $amp_scripts ) ) as $missing_script_handle ) {
			if ( ! wp_script_is( $missing_script_handle, 'registered' ) ) {
				continue;
			}
			$attrs = array(
				'src'   => wp_scripts()->registered[ $missing_script_handle ]->src,
				'async' => '',
			);
			if ( 'amp-mustache' === $missing_script_handle ) {
				$attrs['custom-template'] = $missing_script_handle;
			} else {
				$attrs['custom-element'] = $missing_script_handle;
			}

			$amp_scripts[ $missing_script_handle ] = AMP_DOM_Utils::create_node( $dom, 'script', $attrs );
		}

		/* phpcs:ignore Squiz.PHP.CommentedOutCode.Found
		 *
		 * "Next, preload the AMP runtime v0.js <script> tag with <link as=script href=https://cdn.ampproject.org/v0.js rel=preload>.
		 * The AMP runtime should start downloading as soon as possible because the AMP boilerplate hides the document via body { visibility:hidden }
		 * until the AMP runtime has loaded. Preloading the AMP runtime tells the browser to download the script with a higher priority."
		 * {@link https://docs.google.com/document/d/169XUxtSSEJb16NfkrCr9y5lqhUR7vxXEAsNxBzg07fM/edit#heading=h.2ha259c3ffos Optimize the AMP Runtime loading}
		 */
		$prioritized_preloads = array();
		if ( ! isset( $links['preload'] ) ) {
			$links['preload'] = array();
		}

		$prioritized_preloads[] = AMP_DOM_Utils::create_node( $dom, 'link', array(
			'rel'  => 'preload',
			'as'   => 'script',
			'href' => $runtime_src,
		) );

		$amp_script_handles = array_keys( $amp_scripts );
		foreach ( array_intersect( $render_delaying_extensions, $amp_script_handles ) as $script_handle ) {
			if ( ! in_array( $script_handle, $render_delaying_extensions, true ) ) {
				continue;
			}
			$prioritized_preloads[] = AMP_DOM_Utils::create_node( $dom, 'link', array(
				'rel'  => 'preload',
				'as'   => 'script',
				'href' => $amp_scripts[ $script_handle ]->getAttribute( 'src' ),
			) );
		}
		$links['preload'] = array_merge( $prioritized_preloads, $links['preload'] );

		$link_relations = array( 'preconnect', 'dns-prefetch', 'preload', 'prerender', 'prefetch' );
		foreach ( $link_relations as $rel ) {
			if ( ! isset( $links[ $rel ] ) ) {
				continue;
			}
			foreach ( $links[ $rel ] as $link ) {
				if ( $link->parentNode ) {
					$link->parentNode->removeChild( $link ); // So we can move it.
				}
				$head->insertBefore( $link, $previous_node->nextSibling );
				$previous_node = $link;
			}
		}

		/*
		 * "Specify the <script> tags for render-delaying extensions (e.g., amp-experiment, amp-dynamic-css-classes, and amp-story)."
		 * "Specify the <script> tags for remaining extensions (e.g., amp-bind, ...). These extensions are not render-delaying and therefore
		 * should not be preloaded because they might take away important bandwidth for the initial render."
		 * {@link https://docs.google.com/document/d/169XUxtSSEJb16NfkrCr9y5lqhUR7vxXEAsNxBzg07fM/edit AMP Hosting Guide}
		 */
		if ( isset( $amp_scripts['amp-runtime'] ) ) {
			$ordered_scripts['amp-runtime'] = $amp_scripts['amp-runtime'];
		}
		foreach ( $render_delaying_extensions as $extension ) {
			if ( isset( $amp_scripts[ $extension ] ) ) {
				$ordered_scripts[ $extension ] = $amp_scripts[ $extension ];
				unset( $amp_scripts[ $extension ] );
			}
		}

		$ordered_scripts = array_merge( $ordered_scripts, $amp_scripts );
		foreach ( $ordered_scripts as $ordered_script ) {
			$head->insertBefore( $ordered_script, $previous_node->nextSibling );
			$previous_node = $ordered_script;
		}

		/*
		 * "Specify the <link> tag for your favicon."
		 * {@link https://docs.google.com/document/d/169XUxtSSEJb16NfkrCr9y5lqhUR7vxXEAsNxBzg07fM/edit AMP Hosting Guide}
		 */
		if ( isset( $links['icon'] ) ) {
			foreach ( $links['icon'] as $link ) {
				$link->parentNode->removeChild( $link ); // So we can move it.
				$head->insertBefore( $link, $previous_node->nextSibling );
				$previous_node = $link;
			}
		}

		// Note the style[amp-custom] and style[amp-boilerplate] are output in the add_hooks() method.
		unset( $previous_node );
	}

	/**
	 * Dequeue Customizer assets which are not necessary outside the preview iframe.
	 *
	 * Prevent enqueueing customize-preview styles if not in customizer preview iframe.
	 * These are only needed for when there is live editing of content, such as selective refresh.
	 *
	 * @since 0.7
	 */
	public static function dequeue_customize_preview_scripts() {

		// Dequeue styles unnecessary unless in customizer preview iframe when editing (such as for edit shortcuts).
		if ( ! self::is_customize_preview_iframe() ) {
			wp_dequeue_style( 'customize-preview' );
			foreach ( wp_styles()->registered as $handle => $dependency ) {
				if ( in_array( 'customize-preview', $dependency->deps, true ) ) {
					wp_dequeue_style( $handle );
				}
			}
		}
	}

	/**
	 * Start output buffering.
	 *
	 * @since 0.7
	 * @see AMP_Theme_Support::finish_output_buffering()
	 */
	public static function start_output_buffering() {
		/*
		 * Disable the New Relic Browser agent on AMP responses.
		 * This prevents the New Relic from causing invalid AMP responses due the NREUM script it injects after the meta charset:
		 * https://docs.newrelic.com/docs/browser/new-relic-browser/troubleshooting/google-amp-validator-fails-due-3rd-party-script
		 * Sites with New Relic will need to specially configure New Relic for AMP:
		 * https://docs.newrelic.com/docs/browser/new-relic-browser/installation/monitor-amp-pages-new-relic-browser
		 */
		if ( function_exists( 'newrelic_disable_autorum' ) ) {
			newrelic_disable_autorum();
		}

		ob_start( array( __CLASS__, 'finish_output_buffering' ) );
		self::$is_output_buffering = true;
	}

	/**
	 * Determine whether output buffering has started.
	 *
	 * @since 0.7
	 * @see AMP_Theme_Support::start_output_buffering()
	 * @see AMP_Theme_Support::finish_output_buffering()
	 *
	 * @return bool Whether output buffering has started.
	 */
	public static function is_output_buffering() {
		return self::$is_output_buffering;
	}

	/**
	 * Finish output buffering.
	 *
	 * @since 0.7
	 * @see AMP_Theme_Support::start_output_buffering()
	 *
	 * @param string $response Buffered Response.
	 * @return string Processed Response.
	 */
	public static function finish_output_buffering( $response ) {
		self::$is_output_buffering = false;
		return self::prepare_response( $response );
	}

	/**
	 * Filter rendered partial to convert to AMP.
	 *
	 * @see WP_Customize_Partial::render()
	 *
	 * @param string|mixed $partial Rendered partial.
	 * @return string|mixed Filtered partial.
	 * @global int $content_width
	 */
	public static function filter_customize_partial_render( $partial ) {
		global $content_width;
		if ( is_string( $partial ) && preg_match( '/<\w/', $partial ) ) {
			$dom  = AMP_DOM_Utils::get_dom_from_content( $partial );
			$args = array(
				'content_max_width'    => ! empty( $content_width ) ? $content_width : AMP_Post_Template::CONTENT_MAX_WIDTH, // Back-compat.
				'use_document_element' => false,
				'allow_dirty_styles'   => true,
				'allow_dirty_scripts'  => false,
			);
			AMP_Content_Sanitizer::sanitize_document( $dom, self::$sanitizer_classes, $args ); // @todo Include script assets in response?
			$partial = AMP_DOM_Utils::get_content_from_dom( $dom );
		}
		return $partial;
	}

	/**
	 * Process response to ensure AMP validity.
	 *
	 * @since 0.7
	 *
	 * @param string $response HTML document response. By default it expects a complete document.
	 * @param array  $args     Args to send to the preprocessor/sanitizer.
	 * @return string AMP document response.
	 * @global int $content_width
	 */
	public static function prepare_response( $response, $args = array() ) {
		global $content_width;
		$prepare_response_start = microtime( true );

		if ( isset( $args['validation_error_callback'] ) ) {
			_doing_it_wrong( __METHOD__, 'Do not supply validation_error_callback arg.', '1.0' );
			unset( $args['validation_error_callback'] );
		}

		/*
		 * Check if the response starts with HTML markup.
		 * Without this check, JSON responses will be erroneously corrupted,
		 * being wrapped in HTML documents.
		 */
		if ( '<' !== substr( ltrim( $response ), 0, 1 ) ) {
			return $response;
		}

		// Dependencies on the PWA plugin for service worker streaming.
		$stream_fragment = null;
		if ( class_exists( 'WP_Service_Worker_Navigation_Routing_Component' ) && current_theme_supports( WP_Service_Worker_Navigation_Routing_Component::STREAM_THEME_SUPPORT ) ) {
			$stream_fragment = WP_Service_Worker_Navigation_Routing_Component::get_stream_fragment_query_var();
		}

		$args = array_merge(
			array(
				'content_max_width'       => ! empty( $content_width ) ? $content_width : AMP_Post_Template::CONTENT_MAX_WIDTH, // Back-compat.
				'use_document_element'    => true,
				'allow_dirty_styles'      => self::is_customize_preview_iframe(), // Dirty styles only needed when editing (e.g. for edit shortcodes).
				'allow_dirty_scripts'     => is_customize_preview(), // Scripts are always needed to inject changeset UUID.
				'enable_response_caching' => (
					! ( defined( 'WP_DEBUG' ) && WP_DEBUG )
					&&
					! AMP_Validation_Manager::should_validate_response()
					&&
					! is_customize_preview()
				),
				'user_can_validate'       => AMP_Validation_Manager::has_cap(),
				'stream_fragment'         => $stream_fragment,
			),
			$args
		);

		$current_url = amp_get_current_url();
		$ampless_url = amp_remove_endpoint( $current_url );

		// When response caching is enabled, determine if it should be turned off for cache misses.
		$caches_for_url = null;
		if ( true === $args['enable_response_caching'] ) {
			list( $disable_response_caching, $caches_for_url ) = self::check_for_cache_misses();
			$args['enable_response_caching']                   = ! $disable_response_caching;
		}

		/*
		 * Set response cache hash, the data values dictates whether a new hash key should be generated or not.
		 * This is also used as the ETag.
		 */
		$response_cache_key = md5( wp_json_encode( array(
			$args,
			$response,
			self::$sanitizer_classes,
			self::$embed_handlers,
			AMP__VERSION,
		) ) );

		/*
		 * Per rfc7232:
		 * "The server generating a 304 response MUST generate any of the
		 * following header fields that would have been sent in a 200 (OK)
		 * response to the same request: Cache-Control, Content-Location, Date,
		 * ETag, Expires, and Vary." The only one of these headers which would
		 * not have been set yet during the WordPress template generation is
		 * the ETag. The AMP plugin sends a Vary header at amp_init.
		 */
		AMP_HTTP::send_header( 'ETag', $response_cache_key );

		// Handle responses that are cached by the browser.
		if ( isset( $_SERVER['HTTP_IF_NONE_MATCH'] ) && $_SERVER['HTTP_IF_NONE_MATCH'] === $response_cache_key ) {
			status_header( 304 );
			return '';
		}

		// Return cache if enabled and found.
		$cache_response = null;
		if ( true === $args['enable_response_caching'] ) {
			$response_cache = wp_cache_get( $response_cache_key, self::RESPONSE_CACHE_GROUP );

			// Make sure that all of the validation errors should be sanitized in the same way; if not, then the cached body should be discarded.
			$blocking_error_count = 0;
			if ( isset( $response_cache['validation_results'] ) ) {
				foreach ( $response_cache['validation_results'] as $validation_result ) {
					if ( ! $validation_result['sanitized'] ) {
						$blocking_error_count++;
					}
					$should_sanitize = AMP_Validation_Error_Taxonomy::is_validation_error_sanitized( $validation_result['error'] );
					if ( $should_sanitize !== $validation_result['sanitized'] ) {
						unset( $response_cache['body'] );
						break;
					}
				}
			}

			// Short-circuit response with cached body.
			if ( isset( $response_cache['body'] ) ) {

				// Re-send the headers that were sent before when the response was first cached.
				if ( isset( $response_cache['headers'] ) ) {
					foreach ( $response_cache['headers'] as $header ) {
						if ( in_array( $header, AMP_HTTP::$headers_sent, true ) ) {
							continue; // Skip sending headers that were already sent prior to post-processing.
						}
						AMP_HTTP::send_header( $header['name'], $header['value'], wp_array_slice_assoc( $header, array( 'replace', 'status_code' ) ) );
					}
				}

				AMP_HTTP::send_server_timing( 'amp_processor_cache_hit', -$prepare_response_start );

				// Redirect to non-AMP version.
				if ( ! amp_is_canonical() && $blocking_error_count > 0 ) {
					if ( AMP_Validation_Manager::has_cap() ) {
						$ampless_url = add_query_arg( AMP_Validation_Manager::VALIDATION_ERRORS_QUERY_VAR, $blocking_error_count, $ampless_url );
					}
					wp_safe_redirect( $ampless_url );
				}
				return $response_cache['body'];
			}

			$cache_response = function( $body, $validation_results ) use ( $response_cache_key, $caches_for_url ) {
				$caches_for_url[] = $response_cache_key;
				wp_cache_set(
					AMP_Theme_Support::POST_PROCESSOR_CACHE_EFFECTIVENESS_KEY,
					$caches_for_url,
					AMP_Theme_Support::POST_PROCESSOR_CACHE_EFFECTIVENESS_GROUP,
					600 // 10 minute cache.
				);

				return wp_cache_set(
					$response_cache_key,
					array(
						'headers'            => AMP_HTTP::$headers_sent,
						'body'               => $body,
						'validation_results' => $validation_results,
					),
					AMP_Theme_Support::RESPONSE_CACHE_GROUP,
					MONTH_IN_SECONDS
				);
			};
		}

		AMP_HTTP::send_server_timing( 'amp_output_buffer', -self::$init_start_time, 'AMP Output Buffer' );

		$dom_parse_start = microtime( true );

		/*
		 * Make sure that <meta charset> is present in output prior to parsing.
		 * Note that the meta charset is supposed to appear within the first 1024 bytes.
		 * See <https://www.w3.org/International/questions/qa-html-encoding-declarations>.
		 */
		if ( ! preg_match( '#<meta[^>]+charset=#i', substr( $response, 0, 1024 ) ) ) {
			$response = preg_replace(
				'/(<head[^>]*>)/i',
				'$1' . sprintf( '<meta charset="%s">', esc_attr( get_bloginfo( 'charset' ) ) ),
				$response,
				1
			);
		}

		$dom  = AMP_DOM_Utils::get_dom( $response );
		$head = $dom->getElementsByTagName( 'head' )->item( 0 );

		// Remove scripts that are being added for PWA service worker streaming for restoration later.
		$stream_combine_script_define_element     = null;
		$stream_combine_script_define_placeholder = null;
		$stream_combine_script_invoke_element     = null;
		$stream_combine_script_invoke_placeholder = null;
		if ( 'header' === $stream_fragment ) {
			$stream_combine_script_define_element = $dom->getElementById( WP_Service_Worker_Navigation_Routing_Component::STREAM_COMBINE_DEFINE_SCRIPT_ID );
			if ( $stream_combine_script_define_element ) {
				$stream_combine_script_define_placeholder = $dom->createComment( WP_Service_Worker_Navigation_Routing_Component::STREAM_COMBINE_DEFINE_SCRIPT_ID );
				$stream_combine_script_define_element->parentNode->replaceChild( $stream_combine_script_define_placeholder, $stream_combine_script_define_element );
			}
		} elseif ( 'body' === $stream_fragment ) {
			$stream_combine_script_invoke_placeholder = $dom->getElementById( WP_Service_Worker_Navigation_Routing_Component::STREAM_FRAGMENT_BOUNDARY_ELEMENT_ID );
		}

		// 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 );
				}
			}
		}

		// Make sure scripts from the body get moved to the head.
		if ( isset( $head ) ) {
			$xpath = new DOMXPath( $dom );
			foreach ( $xpath->query( '//body//script[ @custom-element or @custom-template ]' ) as $script ) {
				$head->appendChild( $script->parentNode->removeChild( $script ) );
			}
		}

		// Ensure the mandatory amp attribute is present on the html element.
		if ( ! $dom->documentElement->hasAttribute( 'amp' ) && ! $dom->documentElement->hasAttribute( '⚡️' ) ) {
			$dom->documentElement->setAttribute( 'amp', '' );
		}

		AMP_HTTP::send_server_timing( 'amp_dom_parse', -$dom_parse_start, 'AMP DOM Parse' );

		$assets = AMP_Content_Sanitizer::sanitize_document( $dom, self::$sanitizer_classes, $args );

		// Determine what the validation errors are.
		$blocking_error_count = 0;
		$validation_results   = array();
		foreach ( AMP_Validation_Manager::$validation_results as $validation_result ) {
			if ( ! $validation_result['sanitized'] ) {
				$blocking_error_count++;
			}
			unset( $validation_result['error']['sources'] );
			$validation_results[] = $validation_result;
		}

		$dom_serialize_start = microtime( true );

		// Gather all component scripts that are used in the document and then render any not already printed.
		$amp_scripts = $assets['scripts'];
		foreach ( self::$embed_handlers as $embed_handler ) {
			$amp_scripts = array_merge(
				$amp_scripts,
				$embed_handler->get_scripts()
			);
		}
		foreach ( $amp_scripts as $handle => $src ) {
			/*
			 * Make sure the src is up-to-date. This allows for embed handlers to override the
			 * default extension version by defining a different URL.
			 */
			if ( is_string( $src ) && wp_script_is( $handle, 'registered' ) ) {
				wp_scripts()->registered[ $handle ]->src = $src;
			}
		}

		self::ensure_required_markup( $dom, array_keys( $amp_scripts ) );

		if ( $blocking_error_count > 0 && ! AMP_Validation_Manager::should_validate_response() ) {
			/*
			 * In native AMP, strip html@amp attribute to prevent GSC from complaining about a validation error
			 * already surfaced inside of WordPress. This is intended to not serve dirty AMP, but rather a
			 * non-AMP document (intentionally not valid AMP) that contains the AMP runtime and AMP components.
			 */
			if ( amp_is_canonical() ) {
				$dom->documentElement->removeAttribute( 'amp' );
				$dom->documentElement->removeAttribute( '⚡️' );

				/*
				 * 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 );
				}
			} elseif ( ! self::is_customize_preview_iframe() ) {
				$response = esc_html__( 'Redirecting to non-AMP version.', 'amp' );

				if ( $cache_response ) {
					$cache_response( $response, $validation_results );
				}

				/*
				 * Temporary redirect because AMP URL may return when blocking validation errors
				 * occur or when a non-canonical AMP theme is used.
				 */
				if ( AMP_Validation_Manager::has_cap() ) {
					$ampless_url = add_query_arg( AMP_Validation_Manager::VALIDATION_ERRORS_QUERY_VAR, $blocking_error_count, $ampless_url );
				}
				wp_safe_redirect( $ampless_url, 302 );
				return $response;
			}
		}

		// @todo If 'utf-8' is not the blog charset, then we'll need to do some character encoding conversation or "entityification".
		if ( 'utf-8' !== strtolower( get_bloginfo( 'charset' ) ) ) {
			/* translators: %s: the charset of the current site. */
			trigger_error( esc_html( sprintf( __( 'The database has the %s encoding when it needs to be utf-8 to work with AMP.', 'amp' ), get_bloginfo( 'charset' ) ) ), E_USER_WARNING ); // phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_trigger_error
		}

		AMP_Validation_Manager::finalize_validation( $dom, array(
			'remove_source_comments' => ! isset( $_GET['amp_preserve_source_comments'] ), // WPCS: CSRF.
		) );

		// For service worker streaming, restore the script that was removed above and obtain the script that should be added to the body fragment.
		$truncate_after_comment  = null;
		$truncate_before_comment = null;
		if ( $stream_fragment ) {
			if ( $stream_combine_script_define_placeholder && $stream_combine_script_define_element ) {
				$stream_combine_script_define_placeholder->parentNode->replaceChild( $stream_combine_script_define_element, $stream_combine_script_define_placeholder );
				$truncate_after_comment = $dom->createComment( 'AMP_TRUNCATE_RESPONSE_FOR_STREAM_HEADER' );
				$stream_combine_script_define_element->parentNode->insertBefore( $truncate_after_comment, $stream_combine_script_define_element->nextSibling );
			}
			if ( $stream_combine_script_invoke_placeholder ) {
				$stream_combine_script_invoke_element = WP_Service_Worker_Navigation_Routing_Component::get_header_combine_invoke_script( $dom, false );
				$stream_combine_script_invoke_placeholder->parentNode->replaceChild( $stream_combine_script_invoke_element, $stream_combine_script_invoke_placeholder );
				$truncate_before_comment = $dom->createComment( 'AMP_TRUNCATE_RESPONSE_FOR_STREAM_BODY' );
				$stream_combine_script_invoke_element->parentNode->insertBefore( $truncate_before_comment, $stream_combine_script_invoke_element );
			}
		}

		$response  = "<!DOCTYPE html>\n";
		$response .= AMP_DOM_Utils::get_content_from_dom_node( $dom, $dom->documentElement );

		// For service worker streaming, make sure that the header response doesn't contain closing tags, and that the body fragment starts with the required script tag.
		if ( $truncate_after_comment ) {
			$search   = sprintf( '<!--%s-->', $truncate_after_comment->nodeValue );
			$position = strpos( $response, $search );
			if ( false !== $position ) {
				$response = substr( $response, 0, $position );
			}
		}
		if ( $truncate_before_comment ) {
			$search   = sprintf( '<!--%s-->', $truncate_before_comment->nodeValue );
			$position = strpos( $response, $search );
			if ( false !== $position ) {
				$response = substr( $response, $position + strlen( $search ) );
			}
		}

		AMP_HTTP::send_server_timing( 'amp_dom_serialize', -$dom_serialize_start, 'AMP DOM Serialize' );

		// Cache response if enabled.
		if ( $cache_response ) {
			$cache_response( $response, $validation_results );
		}

		return $response;
	}

	/**
	 * Check for cache misses. When found, store in an option to retain the URL.
	 *
	 * @since 1.0
	 *
	 * @return array {
	 *     State.
	 *
	 *     @type bool       Flag indicating if the threshold has been exceeded.
	 *     @type string[]   Collection of URLs.
	 * }
	 */
	private static function check_for_cache_misses() {
		// If the cache miss threshold is exceeded, return true.
		if ( self::exceeded_cache_miss_threshold() ) {
			return array( true, null );
		}

		// Get the cache miss URLs.
		$cache_miss_urls = wp_cache_get( self::POST_PROCESSOR_CACHE_EFFECTIVENESS_KEY, self::POST_PROCESSOR_CACHE_EFFECTIVENESS_GROUP );
		$cache_miss_urls = is_array( $cache_miss_urls ) ? $cache_miss_urls : array();

		$exceeded_threshold = (
			! empty( $cache_miss_urls )
			&&
			count( $cache_miss_urls ) >= self::CACHE_MISS_THRESHOLD
		);

		if ( ! $exceeded_threshold ) {
			return array( $exceeded_threshold, $cache_miss_urls );
		}

		// When the threshold is exceeded, store the URL for cache miss and turn off response caching.
		update_option( self::CACHE_MISS_URL_OPTION, amp_get_current_url() );
		AMP_Options_Manager::update_option( 'enable_response_caching', false );
		return array( true, null );
	}

	/**
	 * Reset the cache miss URL option.
	 *
	 * @since 1.0
	 */
	public static function reset_cache_miss_url_option() {
		if ( get_option( self::CACHE_MISS_URL_OPTION ) ) {
			delete_option( self::CACHE_MISS_URL_OPTION );
		}
	}

	/**
	 * Checks if cache miss threshold has been exceeded.
	 *
	 * @since 1.0
	 *
	 * @return bool
	 */
	public static function exceeded_cache_miss_threshold() {
		$url = get_option( self::CACHE_MISS_URL_OPTION, false );
		return ! empty( $url );
	}

	/**
	 * Adds 'data-amp-layout' to the allowed <img> attributes for wp_kses().
	 *
	 * @since 0.7
	 *
	 * @param array $context Allowed tags and their allowed attributes.
	 * @return array $context Filtered allowed tags and attributes.
	 */
	public static function whitelist_layout_in_wp_kses_allowed_html( $context ) {
		if ( ! empty( $context['img']['width'] ) && ! empty( $context['img']['height'] ) ) {
			$context['img']['data-amp-layout'] = true;
		}

		return $context;
	}

	/**
	 * Enqueue AMP assets if this is an AMP endpoint.
	 *
	 * @since 0.7
	 *
	 * @return void
	 */
	public static function enqueue_assets() {
		wp_enqueue_script( 'amp-runtime' );

		// Enqueue default styles expected by sanitizer.
		wp_enqueue_style( 'amp-default', amp_get_asset_url( 'css/amp-default.css' ), array(), AMP__VERSION );
	}

	/**
	 * Conditionally replace the header image markup with a header video or image.
	 *
	 * This is JS-driven in Core themes like Twenty Sixteen and Twenty Seventeen.
	 * So in order for the header video to display, this replaces the markup of the header image.
	 *
	 * @since 1.0
	 * @link https://github.com/WordPress/wordpress-develop/blob/d002fde80e5e3a083e5f950313163f566561517f/src/wp-includes/js/wp-custom-header.js#L54
	 * @link https://github.com/WordPress/wordpress-develop/blob/d002fde80e5e3a083e5f950313163f566561517f/src/wp-includes/js/wp-custom-header.js#L78
	 *
	 * @param string $image_markup The image markup to filter.
	 * @return string $html Filtered markup.
	 */
	public static function amend_header_image_with_video_header( $image_markup ) {

		// If there is no video, just pass the image through.
		if ( ! has_header_video() || ! is_header_video_active() ) {
			return $image_markup;
		};

		$video_settings   = get_header_video_settings();
		$parsed_url       = wp_parse_url( $video_settings['videoUrl'] );
		$query            = isset( $parsed_url['query'] ) ? wp_parse_args( $parsed_url['query'] ) : array();
		$video_attributes = array(
			'media'    => '(min-width: ' . $video_settings['minWidth'] . 'px)',
			'width'    => $video_settings['width'],
			'height'   => $video_settings['height'],
			'layout'   => 'responsive',
			'autoplay' => '',
			'id'       => 'wp-custom-header-video',
		);

		$youtube_id = null;
		if ( isset( $parsed_url['host'] ) && preg_match( '/(^|\.)(youtube\.com|youtu\.be)$/', $parsed_url['host'] ) ) {
			if ( 'youtu.be' === $parsed_url['host'] && ! empty( $parsed_url['path'] ) ) {
				$youtube_id = trim( $parsed_url['path'], '/' );
			} elseif ( isset( $query['v'] ) ) {
				$youtube_id = $query['v'];
			}
		}

		// If the video URL is for YouTube, return an <amp-youtube> element.
		if ( ! empty( $youtube_id ) ) {
			$video_markup = AMP_HTML_Utils::build_tag(
				'amp-youtube',
				array_merge(
					$video_attributes,
					array(
						'data-videoid'        => $youtube_id,
						'data-param-rel'      => '0', // Don't show related videos.
						'data-param-showinfo' => '0', // Don't show video title at the top.
						'data-param-controls' => '0', // Don't show video controls.
					)
				)
			);
		} else {
			$video_markup = AMP_HTML_Utils::build_tag(
				'amp-video',
				array_merge(
					$video_attributes,
					array(
						'src' => $video_settings['videoUrl'],
					)
				)
			);
		}

		return $image_markup . $video_markup;
	}
}