Provide fallback JPEG images in frontend when WebP is not supported by the browser

modules/images/webp-uploads/fallback.js

@@ -0,0 +1,79 @@
+window.wpPerfLab = window.wpPerfLab || {};
+
+( function( document ) {
+	window.wpPerfLab.webpUploadsFallbackWebpImages = function( media ) {
+		for ( var i = 0; i < media.length; i++ ) {
+			try {
+				if ( ! media[i].media_details.sources || ! media[i].media_details.sources['image/jpeg'] ) {
+					continue;
+				}
+
+				var ext = media[i].media_details.sources['image/jpeg'].file.match( /\.\w+$/i );
+				if ( ! ext || ! ext[0] ) {
+					continue;
+				}
+
+				var images = document.querySelectorAll( 'img.wp-image-' + media[i].id );
+				for ( var j = 0; j < images.length; j++ ) {
+					images[j].src = images[j].src.replace( /\.webp$/i, ext[0] );
+					var srcset = images[j].getAttribute( 'srcset' );
+					if ( srcset ) {
+						images[j].setAttribute( 'srcset', srcset.replace( /\.webp(\s)/ig, ext[0] + '$1' ) );
+					}
+				}
+			} catch ( e ) {
+			}
+		}
+	};
+
+	var restApi = document.getElementById( 'webpUploadsFallbackWebpImages' ).getAttribute( 'data-rest-api' );
+
+	var loadMediaDetails = function( nodes ) {
+		var ids = [];
+		for ( var i = 0; i < nodes.length; i++ ) {
+			var node = nodes[i];
+			var srcset = node.getAttribute( 'srcset' ) || '';
+
+			if (
+				node.nodeName !== "IMG" ||
+				( ! node.src.match( /\.webp$/i ) && ! srcset.match( /\.webp\s+/ ) )
+			) {
+				continue;
+			}
+
+			var attachment = node.className.match( /wp-image-(\d+)/i );
+			if ( attachment && attachment[1] && ids.indexOf( attachment[1] ) === -1 ) {
+				ids.push( attachment[1] );
+			}
+		}
+
+		for ( var page = 0, pages = Math.ceil( ids.length / 100 ); page < pages; page++ ) {
+			var pageIds = [];
+			for ( var i = 0; i < 100 && i + page * 100 < ids.length; i++ ) {
+				pageIds.push( ids[ i + page * 100 ] );
+			}
+
+			var jsonp = document.createElement( 'script' );
+			jsonp.src = restApi + 'wp/v2/media/?_fields=id,media_details&_jsonp=wpPerfLab.webpUploadsFallbackWebpImages&per_page=100&include=' + pageIds.join( ',' );
+			document.body.appendChild( jsonp );
+		}
+	};
+
+	try {
+		// Loop through already available images.
+		loadMediaDetails( document.querySelectorAll( 'img' ) );
+
+		// Start the mutation observer to update images added dynamically.
+		var observer = new MutationObserver( function( mutationList ) {
+			for ( var i = 0; i < mutationList.length; i++ ) {
+				loadMediaDetails( mutationList[i].addedNodes );
+			}
+		} );
+
+		observer.observe( document.body, {
+			subtree: true,
+			childList: true,
+		} );
+	} catch ( e ) {
+	}
+} )( document );

modules/images/webp-uploads/load.php

@@ -482,12 +482,13 @@ function webp_uploads_update_image_references( $content ) {
  *
  * @since 1.0.0
  *
- * @param string $image         An <img> tag where the urls would be updated.
- * @param string $context       The context where this is function is being used.
- * @param int    $attachment_id The ID of the attachment being modified.
+ * @param string $original_image An <img> tag where the urls would be updated.
+ * @param string $context        The context where this is function is being used.
+ * @param int    $attachment_id  The ID of the attachment being modified.
  * @return string The updated img tag.
  */
-function webp_uploads_img_tag_update_mime_type( $image, $context, $attachment_id ) {
+function webp_uploads_img_tag_update_mime_type( $original_image, $context, $attachment_id ) {
+	$image    = $original_image;
 	$metadata = wp_get_attachment_metadata( $attachment_id );
 	if ( empty( $metadata['file'] ) ) {
 		return $image;
@@ -621,6 +622,16 @@ function webp_uploads_img_tag_update_mime_type( $image, $context, $attachment_id
 		}
 	}
 
+	if (
+		! has_action( 'wp_footer', 'webp_uploads_wepb_fallback' ) &&
+		$image !== $original_image &&
+		'the_content' === $context &&
+		'image/jpeg' === $original_mime &&
+		'image/webp' === $target_mime
+	) {
+		add_action( 'wp_footer', 'webp_uploads_wepb_fallback' );
+	}
+
 	return $image;
 }
 
@@ -640,6 +651,51 @@ function webp_uploads_update_featured_image( $html, $post_id, $attachment_id ) {
 }
 add_filter( 'post_thumbnail_html', 'webp_uploads_update_featured_image', 10, 3 );
 
+/**
+ * Adds a fallback mechanism to replace webp images with jpeg alternatives on older browsers.
+ *
+ * @since n.e.x.t
+ */
+function webp_uploads_wepb_fallback() {
+	// Get mime type transofrms for the site.
+	$transforms = webp_uploads_get_upload_image_mime_transforms();
+
+	// We need to add fallback only if jpeg alternatives for the webp images are enabled for the server.
+	$preserve_jpegs_for_jpeg_transforms = isset( $transforms['image/jpeg'] ) && in_array( 'image/jpeg', $transforms['image/jpeg'], true ) && in_array( 'image/webp', $transforms['image/jpeg'], true );
+	$preserve_jpegs_for_webp_transforms = isset( $transforms['image/webp'] ) && in_array( 'image/jpeg', $transforms['image/webp'], true ) && in_array( 'image/webp', $transforms['image/webp'], true );
+	if ( ! $preserve_jpegs_for_jpeg_transforms && ! $preserve_jpegs_for_webp_transforms ) {
+		return;
+	}
+
+	ob_start();
+
+	?>
+	( function( d, i, s, p ) {
+		s = d.createElement( s );
+		s.src = '<?php echo esc_url_raw( plugins_url( '/fallback.js', __FILE__ ) ); ?>';
+
+		i = d.createElement( i );
+		i.src = p + 'jIAAABXRUJQVlA4ICYAAACyAgCdASoCAAEALmk0mk0iIiIiIgBoSygABc6zbAAA/v56QAAAAA==';
+		i.onload = function() {
+			i.src = p + 'h4AAABXRUJQVlA4TBEAAAAvAQAAAAfQ//73v/+BiOh/AAA=';
+		};
+
+		i.onerror = function() {
+			d.body.appendChild( s );
+		};
+	} )( document, 'img', 'script', 'data:image/webp;base64,UklGR' );
+	<?php
+	$javascript = ob_get_clean();
+
+	wp_print_inline_script_tag(
+		preg_replace( '/\s+/', '', $javascript ),
+		array(
+			'id'            => 'webpUploadsFallbackWebpImages',
+			'data-rest-api' => esc_url_raw( trailingslashit( get_rest_url() ) ),
+		)
+	);
+}
+
 /**
  * Returns an array of image size names that have secondary mime type output enabled. Core sizes and
  * core theme sizes are enabled by default.

tests/modules/images/webp-uploads/load-tests.php

@@ -636,6 +636,44 @@ function() {
 		$this->assertNotSame( $tag, webp_uploads_img_tag_update_mime_type( $tag, 'the_content', $attachment_id ) );
 	}
 
+	/**
+	 * Tests that the fallback script is added when a post with updated images is rendered.
+	 *
+	 * @test
+	 */
+	public function it_should_add_fallback_script_if_content_has_updated_images() {
+		$attachment_id = $this->factory->attachment->create_upload_object(
+			TESTS_PLUGIN_DIR . '/tests/testdata/modules/images/leafs.jpg'
+		);
+
+		apply_filters(
+			'the_content',
+			sprintf(
+				'<p>before image</p>%s<p>after image</p>',
+				wp_get_attachment_image( $attachment_id, 'medium', false, array( 'class' => "wp-image-{$attachment_id}" ) )
+			)
+		);
+
+		$this->assertTrue( has_action( 'wp_footer', 'webp_uploads_wepb_fallback' ) === 10 );
+
+		$footer = get_echo( 'wp_footer' );
+		$this->assertStringContainsString( 'data:image/webp;base64,UklGR', $footer );
+	}
+
+	/**
+	 * Tests that the fallback script is not added when a post with no updated images is rendered.
+	 *
+	 * @test
+	 */
+	public function it_should_not_add_fallback_script_if_content_has_no_updated_images() {
+		apply_filters( 'the_content', '<p>no image</p>' );
+
+		$this->assertFalse( has_action( 'wp_footer', 'webp_uploads_wepb_fallback' ) );
+
+		$footer = get_echo( 'wp_footer' );
+		$this->assertStringNotContainsString( 'data:image/webp;base64,UklGR', $footer );
+	}
+
 	/**
 	 * Tests whether additional mime types generated only for allowed image sizes or not when the filter is used.
 	 *