Preload image URLs for LCP elements with external background images

plugins/embed-optimizer/detect.js

@@ -51,7 +51,7 @@ const loadedElementContentRects = new Map();
  * @type {InitializeCallback}
  * @param {InitializeArgs} args Args.
  */
-export function initialize( { isDebug } ) {
+export async function initialize( { isDebug } ) {
 	/** @type NodeListOf<HTMLDivElement> */
 	const embedWrappers = document.querySelectorAll(
 		'.wp-block-embed > .wp-block-embed__wrapper[data-od-xpath]'

plugins/image-prioritizer/class-image-prioritizer-background-image-styled-tag-visitor.php

@@ -14,11 +14,25 @@
 /**
  * Tag visitor that optimizes elements with background-image styles.
  *
+ * @phpstan-type LcpElementExternalBackgroundImage array{
+ *     url: non-empty-string,
+ *     tag: non-empty-string,
+ *     id: string|null,
+ *     class: string|null,
+ * }
+ *
  * @since 0.1.0
  * @access private
  */
 final class Image_Prioritizer_Background_Image_Styled_Tag_Visitor extends Image_Prioritizer_Tag_Visitor {
 
+	/**
+	 * Tuples of URL Metric group and the common LCP element external background image.
+	 *
+	 * @var array<array{OD_URL_Metric_Group, LcpElementExternalBackgroundImage}>
+	 */
+	private $group_common_lcp_element_external_background_images;
+
 	/**
 	 * Visits a tag.
 	 *
@@ -49,28 +63,121 @@ public function __invoke( OD_Tag_Visitor_Context $context ): bool {
 		}
 
 		if ( is_null( $background_image_url ) ) {
+			$this->maybe_preload_external_lcp_background_image( $context );
 			return false;
 		}
 
 		$xpath = $processor->get_xpath();
 
 		// If this element is the LCP (for a breakpoint group), add a preload link for it.
 		foreach ( $context->url_metric_group_collection->get_groups_by_lcp_element( $xpath ) as $group ) {
-			$link_attributes = array(
+			$this->add_preload_link( $context->link_collection, $group, $background_image_url );
+		}
+
+		return true;
+	}
+
+	/**
+	 * Gets the common LCP element external background image for a URL Metric group.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param OD_URL_Metric_Group $group Group.
+	 * @return LcpElementExternalBackgroundImage|null
+	 */
+	private function get_common_lcp_element_external_background_image( OD_URL_Metric_Group $group ): ?array {
+
+		// If the group is not fully populated, we don't have enough URL Metrics to reliably know whether the background image is consistent across page loads.
+		// This is intentionally not using $group->is_complete() because we still will use stale URL Metrics in the calculation.
+		if ( $group->count() !== $group->get_sample_size() ) {
+			return null;
+		}
+
+		$previous_lcp_element_external_background_image = null;
+		foreach ( $group as $url_metric ) {
+			/**
+			 * Stored data.
+			 *
+			 * @var LcpElementExternalBackgroundImage|null $lcp_element_external_background_image
+			 */
+			$lcp_element_external_background_image = $url_metric->get( 'lcpElementExternalBackgroundImage' );
+			if ( ! is_array( $lcp_element_external_background_image ) ) {
+				return null;
+			}
+			if ( null !== $previous_lcp_element_external_background_image && $previous_lcp_element_external_background_image !== $lcp_element_external_background_image ) {
+				return null;
+			}
+			$previous_lcp_element_external_background_image = $lcp_element_external_background_image;
+		}
+
+		return $previous_lcp_element_external_background_image;
+	}
+
+	/**
+	 * Maybe preloads external background image.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param OD_Tag_Visitor_Context $context Context.
+	 */
+	private function maybe_preload_external_lcp_background_image( OD_Tag_Visitor_Context $context ): void {
+		// Gather the tuples of URL Metric group and the common LCP element external background image.
+		if ( ! is_array( $this->group_common_lcp_element_external_background_images ) ) {
+			$this->group_common_lcp_element_external_background_images = array();
+			foreach ( $context->url_metric_group_collection as $group ) {
+				$common = $this->get_common_lcp_element_external_background_image( $group );
+				if ( is_array( $common ) ) {
+					$this->group_common_lcp_element_external_background_images[] = array( $group, $common );
+				}
+			}
+		}
+
+		// There are no common LCP background images, so abort.
+		if ( count( $this->group_common_lcp_element_external_background_images ) === 0 ) {
+			return;
+		}
+
+		$processor = $context->processor;
+		$tag_name  = strtoupper( (string) $processor->get_tag() );
+		foreach ( $this->group_common_lcp_element_external_background_images as $i => list( $group, $common ) ) {
+			if (
+				// Note that the browser may send a lower-case tag name in the case of XHTML or embedded SVG/MathML, but
+				// the HTML Tag Processor is currently normalizing to all upper-case. The HTML Processor on the other
+				// hand may return the expected case.
+				strtoupper( $common['tag'] ) === $tag_name
+				&&
+				$processor->get_attribute( 'id' ) === $common['id'] // May be checking equality with null.
+				&&
+				$processor->get_attribute( 'class' ) === $common['class'] // May be checking equality with null.
+			) {
+				$this->add_preload_link( $context->link_collection, $group, $common['url'] );
+
+				// Now that the preload link has been added, eliminate the entry to stop looking for it while iterating over the rest of the document.
+				unset( $this->group_common_lcp_element_external_background_images[ $i ] );
+			}
+		}
+	}
+
+	/**
+	 * Adds an image preload link for the group.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param OD_Link_Collection  $link_collection Link collection.
+	 * @param OD_URL_Metric_Group $group           URL Metric group.
+	 * @param non-empty-string    $url             Image URL.
+	 */
+	private function add_preload_link( OD_Link_Collection $link_collection, OD_URL_Metric_Group $group, string $url ): void {
+		$link_collection->add_link(
+			array(
 				'rel'           => 'preload',
 				'fetchpriority' => 'high',
 				'as'            => 'image',
-				'href'          => $background_image_url,
+				'href'          => $url,
 				'media'         => 'screen',
-			);
-
-			$context->link_collection->add_link(
-				$link_attributes,
-				$group->get_minimum_viewport_width(),
-				$group->get_maximum_viewport_width()
-			);
-		}
-
-		return true;
+			),
+			$group->get_minimum_viewport_width(),
+			$group->get_maximum_viewport_width()
+		);
 	}
 }

plugins/image-prioritizer/detect.js

@@ -0,0 +1,206 @@
+/**
+ * Image Prioritizer module for Optimization Detective
+ *
+ * This extension to Optimization Detective captures the LCP element's CSS background image which is not defined with
+ * an inline style attribute but rather in either an external stylesheet loaded with a LINK tag or by stylesheet in
+ * a STYLE element. The URL for this LCP background image and the tag's name, ID, and class are all amended to the
+ * stored URL Metric so that a responsive preload link with fetchpriority=high will be added for that background image
+ * once a URL Metric group is fully populated with URL Metrics that all agree on that being the LCP image, and if the
+ * document has a tag with the same name, ID, and class.
+ */
+
+const consoleLogPrefix = '[Image Prioritizer]';
+
+/**
+ * Detected LCP external background image candidates.
+ *
+ * @type {Array<{url: string, tag: string, id: string, class: string}>}
+ */
+const externalBackgroundImages = [];
+
+/**
+ * @typedef {import("web-vitals").LCPMetric} LCPMetric
+ * @typedef {import("../optimization-detective/types.ts").InitializeCallback} InitializeCallback
+ * @typedef {import("../optimization-detective/types.ts").InitializeArgs} InitializeArgs
+ * @typedef {import("../optimization-detective/types.ts").FinalizeArgs} FinalizeArgs
+ * @typedef {import("../optimization-detective/types.ts").FinalizeCallback} FinalizeCallback
+ */
+
+/**
+ * Logs a message.
+ *
+ * @since n.e.x.t
+ *
+ * @param {...*} message
+ */
+function log( ...message ) {
+	// eslint-disable-next-line no-console
+	console.log( consoleLogPrefix, ...message );
+}
+
+/**
+ * Logs a warning.
+ *
+ * @since n.e.x.t
+ *
+ * @param {...*} message
+ */
+function warn( ...message ) {
+	// eslint-disable-next-line no-console
+	console.warn( consoleLogPrefix, ...message );
+}
+
+/**
+ * Initializes extension.
+ *
+ * @since n.e.x.t
+ *
+ * @type {InitializeCallback}
+ * @param {InitializeArgs} args Args.
+ */
+export async function initialize( { isDebug, webVitalsLibrarySrc } ) {
+	const { onLCP } = await import( webVitalsLibrarySrc );
+	onLCP(
+		( /** @type {LCPMetric} */ metric ) => {
+			handleLCPMetric( metric, isDebug );
+		},
+		{
+			// This avoids needing to click to finalize LCP candidate. While this is helpful for testing, it also
+			// ensures that we always get an LCP candidate reported. Otherwise, the callback may never fire if the
+			// user never does a click or keydown, per <https://github.com/GoogleChrome/web-vitals/blob/07f6f96/src/onLCP.ts#L99-L107>.
+			reportAllChanges: true,
+		}
+	);
+}
+
+/**
+ * Gets the performance resource entry for a given URL.
+ *
+ * @since n.e.x.t
+ *
+ * @param {string} url - Resource URL.
+ * @return {PerformanceResourceTiming|null} Resource entry or null.
+ */
+function getPerformanceResourceByURL( url ) {
+	const entries =
+		/** @type PerformanceResourceTiming[] */ performance.getEntriesByType(
+			'resource'
+		);
+	for ( const entry of entries ) {
+		if ( entry.name === url ) {
+			return entry;
+		}
+	}
+	return null;
+}
+
+/**
+ * Handles a new LCP metric being reported.
+ *
+ * @since n.e.x.t
+ *
+ * @param {LCPMetric} metric  - LCP Metric.
+ * @param {boolean}   isDebug - Whether in debug mode.
+ */
+function handleLCPMetric( metric, isDebug ) {
+	for ( const entry of metric.entries ) {
+		// Look only for LCP entries that have a URL and a corresponding element which is not an IMG or VIDEO.
+		if (
+			! entry.url ||
+			! ( entry.element instanceof HTMLElement ) ||
+			entry.element instanceof HTMLImageElement ||
+			entry.element instanceof HTMLVideoElement
+		) {
+			continue;
+		}
+
+		// Always ignore data: URLs.
+		if ( entry.url.startsWith( 'data:' ) ) {
+			continue;
+		}
+
+		// Skip elements that have the background image defined inline.
+		// These are handled by Image_Prioritizer_Background_Image_Styled_Tag_Visitor.
+		if ( entry.element.style.backgroundImage ) {
+			continue;
+		}
+
+		// Now only consider proceeding with the URL if its loading was initiated with stylesheet or preload link.
+		const resourceEntry = getPerformanceResourceByURL( entry.url );
+		if (
+			! resourceEntry ||
+			! [ 'css', 'link' ].includes( resourceEntry.initiatorType )
+		) {
+			if ( isDebug ) {
+				warn(
+					`Skipped considering URL (${ entry.url }) due to unexpected performance resource timing entry:`,
+					resourceEntry
+				);
+			}
+			return;
+		}
+
+		// Skip URLs that are excessively long. This is the maxLength defined in image_prioritizer_add_element_item_schema_properties().
+		if ( entry.url.length > 500 ) {
+			if ( isDebug ) {
+				log( `Skipping very long URL: ${ entry.url }` );
+			}
+			return;
+		}
+
+		// Note that getAttribute() is used instead of properties so that null can be returned in case of an absent attribute.
+		let id = entry.element.getAttribute( 'id' );
+		if ( null !== id ) {
+			id = id.substring( 0, 100 ); // This is the maxLength defined in image_prioritizer_add_element_item_schema_properties().
+		}
+		let className = entry.element.getAttribute( 'class' );
+		if ( null !== className ) {
+			className = className.substring( 0, 500 ); // This is the maxLength defined in image_prioritizer_add_element_item_schema_properties().
+		}
+
+		// The id and className allow the tag visitor to detect whether the element is still in the document.
+		// This is used instead of having a full XPath which is likely not available since the tag visitor would not
+		// know to return true for this element since it has no awareness of which elements have external backgrounds.
+		const externalBackgroundImage = {
+			url: entry.url,
+			tag: entry.element.tagName,
+			id,
+			class: className,
+		};
+
+		if ( isDebug ) {
+			log(
+				'Detected external LCP background image:',
+				externalBackgroundImage
+			);
+		}
+
+		externalBackgroundImages.push( externalBackgroundImage );
+	}
+}
+
+/**
+ * Finalizes extension.
+ *
+ * @since n.e.x.t
+ *
+ * @type {FinalizeCallback}
+ * @param {FinalizeArgs} args Args.
+ */
+export async function finalize( { extendRootData, isDebug } ) {
+	if ( externalBackgroundImages.length === 0 ) {
+		return;
+	}
+
+	// Get the last detected external background image which is going to be for the LCP element (or very likely will be).
+	const lcpElementExternalBackgroundImage = externalBackgroundImages.pop();
+
+	if ( isDebug ) {
+		log(
+			'Sending external background image for LCP element:',
+			lcpElementExternalBackgroundImage
+		);
+	}
+
+	extendRootData( { lcpElementExternalBackgroundImage } );
+}