Script loader: enable resource preloading with rel='preload'.

Add a `wp_preload_resources` filter that developers can use to add resource preloading.

Preloading helps the browser discover and prioritize important resources earlier during the page load. This ensures that they are available earlier and are less likely to block the page's render.

Props nico23, swissspidy, igrigorik, westonruter, azaozz, furi3r, aristath, spacedmonkey, peterwilsoncc, mihai2u, gziolo. 
Fixes #42438.


Built from https://develop.svn.wordpress.org/trunk@53846


git-svn-id: http://core.svn.wordpress.org/trunk@53405 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/default-filters.php

@@ -324,6 +324,7 @@ add_filter( 'rest_authentication_errors', 'rest_cookie_check_errors', 100 );
 add_action( 'wp_head', '_wp_render_title_tag', 1 );
 add_action( 'wp_head', 'wp_enqueue_scripts', 1 );
 add_action( 'wp_head', 'wp_resource_hints', 2 );
+add_action( 'wp_head', 'wp_preload_resources', 1 );
 add_action( 'wp_head', 'feed_links', 2 );
 add_action( 'wp_head', 'feed_links_extra', 3 );
 add_action( 'wp_head', 'rsd_link' );

wp-includes/general-template.php

@@ -3423,6 +3423,120 @@ function wp_resource_hints() {
 	}
 }
 
+/**
+ * Prints resource preloads directives to browsers.
+ *
+ * Gives directive to browsers to preload specific resources that website will
+ * need very soon, this ensures that they are available earlier and are less
+ * likely to block the page's render. Preload directives should not be used for
+ * non-render-blocking elements, as then they would compete with the
+ * render-blocking ones, slowing down the render.
+ *
+ * These performance improving indicators work by using `<link rel="preload">`.
+ *
+ * @link https://developer.mozilla.org/en-US/docs/Web/HTML/Link_types/preload
+ * @link https://web.dev/preload-responsive-images/
+ *
+ * @since 6.1.0
+ */
+function wp_preload_resources() {
+	/**
+	 * Filters domains and URLs for resource preloads.
+	 *
+	 * @since 6.1.0
+	 *
+	 * @param array  $preload_resources {
+	 *     Array of resources and their attributes, or URLs to print for resource preloads.
+	 *
+	 *     @type array ...$0 {
+	 *         Array of resource attributes.
+	 *
+	 *         @type string $href        URL to include in resource preloads. Required.
+	 *         @type string $as          How the browser should treat the resource
+	 *                                   (`script`, `style`, `image`, `document`, etc).
+	 *         @type string $crossorigin Indicates the CORS policy of the specified resource.
+	 *         @type string $type        Type of the resource (`text/html`, `text/css`, etc).
+	 *         @type string $media       Accepts media types or media queries. Allows responsive preloading.
+	 *         @type string $imagesizes  Responsive source size to the source Set.
+	 *         @type string $imagesrcset Responsive image sources to the source set.
+	 *     }
+	 * }
+	 */
+	$preload_resources = apply_filters( 'wp_preload_resources', array() );
+
+	if ( ! is_array( $preload_resources ) ) {
+		return;
+	}
+
+	$unique_resources = array();
+
+	// Parse the complete resource list and extract unique resources.
+	foreach ( $preload_resources as $resource ) {
+		if ( ! is_array( $resource ) ) {
+			continue;
+		}
+
+		$attributes = $resource;
+		if ( isset( $resource['href'] ) ) {
+			$href = $resource['href'];
+			if ( isset( $unique_resources[ $href ] ) ) {
+				continue;
+			}
+			$unique_resources[ $href ] = $attributes;
+			// Media can use imagesrcset and not href.
+		} elseif ( ( 'image' === $resource['as'] ) &&
+			( isset( $resource['imagesrcset'] ) || isset( $resource['imagesizes'] ) )
+		) {
+			if ( isset( $unique_resources[ $resource['imagesrcset'] ] ) ) {
+				continue;
+			}
+			$unique_resources[ $resource['imagesrcset'] ] = $attributes;
+		} else {
+			continue;
+		}
+	}
+
+	// Build and output the HTML for each unique resource.
+	foreach ( $unique_resources as $unique_resource ) {
+		$html = '';
+
+		foreach ( $unique_resource as $resource_key => $resource_value ) {
+			if ( ! is_scalar( $resource_value ) ) {
+				continue;
+			}
+
+			// Ignore non-supported attributes.
+			$non_supported_attributes = array( 'as', 'crossorigin', 'href', 'imagesrcset', 'imagesizes', 'type', 'media' );
+			if ( ! in_array( $resource_key, $non_supported_attributes, true ) && ! is_numeric( $resource_key ) ) {
+				continue;
+			}
+
+			// imagesrcset only usable when preloading image, ignore otherwise.
+			if ( ( 'imagesrcset' === $resource_key ) && ( ! isset( $unique_resource['as'] ) || ( 'image' !== $unique_resource['as'] ) ) ) {
+				continue;
+			}
+
+			// imagesizes only usable when preloading image and imagesrcset present, ignore otherwise.
+			if ( ( 'imagesizes' === $resource_key ) &&
+				( ! isset( $unique_resource['as'] ) || ( 'image' !== $unique_resource['as'] ) || ! isset( $unique_resource['imagesrcset'] ) )
+			) {
+				continue;
+			}
+
+			$resource_value = ( 'href' === $resource_key ) ? esc_url( $resource_value, array( 'http', 'https' ) ) : esc_attr( $resource_value );
+
+			if ( ! is_string( $resource_key ) ) {
+				$html .= " $resource_value";
+			} else {
+				$html .= " $resource_key='$resource_value'";
+			}
+		}
+		$html = trim( $html );
+
+		printf( "<link rel='preload' %s />\n", $html );
+	}
+}
+
 /**
  * Retrieves a list of unique hosts of all enqueued scripts and styles.
  *

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53845';
+$wp_version = '6.1-alpha-53846';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.