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

@@ -14,6 +14,13 @@
 /**
  * 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
  */
@@ -35,6 +42,14 @@ final class Image_Prioritizer_Background_Image_Styled_Tag_Visitor extends Image_
 	 */
 	private $added_lazy_assets = false;
 
+	/**
+	 * Tuples of URL Metric group and the common LCP element external background image.
+	 *
+	 * @since n.e.x.t
+	 * @var array<array{OD_URL_Metric_Group, LcpElementExternalBackgroundImage}>
+	 */
+	private $group_common_lcp_element_external_background_images;
+
 	/**
 	 * Visits a tag.
 	 *
@@ -65,33 +80,128 @@ 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(
-				'rel'           => 'preload',
-				'fetchpriority' => 'high',
-				'as'            => 'image',
-				'href'          => $background_image_url,
-				'media'         => 'screen',
-			);
-
-			$context->link_collection->add_link(
-				$link_attributes,
-				$group->get_minimum_viewport_width(),
-				$group->get_maximum_viewport_width()
-			);
+			$this->add_image_preload_link( $context->link_collection, $group, $background_image_url );
 		}
 
 		$this->lazy_load_bg_images( $context );
 
 		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.
+		// Note the groups of URL Metrics do not change across invocations, we just need to compute this once for all.
+		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 ( array_keys( $this->group_common_lcp_element_external_background_images ) as $i ) {
+			list( $group, $common ) = $this->group_common_lcp_element_external_background_images[ $i ];
+			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_image_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_image_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'          => $url,
+				'media'         => 'screen',
+			),
+			$group->get_minimum_viewport_width(),
+			$group->get_maximum_viewport_width()
+		);
+	}
+
 	/**
 	 * Optimizes an element with a background image based on whether it is displayed in any initial viewport.
 	 *