Ticket #44427: 44427.10.diff

src/wp-includes/default-filters.php

@@ -175 +175 @@
 add_filter( 'the_content', 'wpautop' );
 add_filter( 'the_content', 'shortcode_unautop' );
 add_filter( 'the_content', 'prepend_attachment' );
-add_filter( 'the_content', 'wp_make_content_images_responsive' );
+add_filter( 'the_content', 'wp_filter_content_tags' );
 
 add_filter( 'the_excerpt', 'wptexturize' );
 add_filter( 'the_excerpt', 'convert_smilies' );
@@ -182 +182 @@
 add_filter( 'the_excerpt', 'convert_chars' );
 add_filter( 'the_excerpt', 'wpautop' );
 add_filter( 'the_excerpt', 'shortcode_unautop' );
+add_filter( 'the_excerpt', 'wp_filter_content_tags' );
 add_filter( 'get_the_excerpt', 'wp_trim_excerpt', 10, 2 );
 
 add_filter( 'the_post_thumbnail_caption', 'wptexturize' );
@@ -207 +208 @@
 add_filter( 'widget_text_content', 'convert_smilies', 20 );
 add_filter( 'widget_text_content', 'wpautop' );
 add_filter( 'widget_text_content', 'shortcode_unautop' );
+add_filter( 'widget_text_content', 'wp_filter_content_tags' );
 add_filter( 'widget_text_content', 'do_shortcode', 11 ); // Runs after wpautop(); note that $post global will be null when shortcodes run.
 
 // RSS filters.

src/wp-includes/deprecated.php

@@ -2509 +2509 @@
  * Determines whether the current admin page is generated by a plugin.
  *
  * Use global $plugin_page and/or get_plugin_page_hookname() hooks.
- * 
+ *
  * For more information on this and similar theme functions, check out
- * the {@link https://developer.wordpress.org/themes/basics/conditional-tags/ 
+ * the {@link https://developer.wordpress.org/themes/basics/conditional-tags/
  * Conditional Tags} article in the Theme Developer Handbook.
- * 
+ *
  * @since 1.5.0
  * @deprecated 3.1.0
  *
@@ -3716 +3716 @@
 
 /**
  * Determines whether the current URL is within the comments popup window.
- * 
+ *
  * For more information on this and similar theme functions, check out
- * the {@link https://developer.wordpress.org/themes/basics/conditional-tags/ 
+ * the {@link https://developer.wordpress.org/themes/basics/conditional-tags/
  * Conditional Tags} article in the Theme Developer Handbook.
- * 
+ *
  * @since 1.5.0
  * @deprecated 4.5.0
  *
@@ -3950 +3950 @@
                 wp_send_json_error( array( 'errorMessage' => __( 'The Press This plugin is required.' ) ) );
         }
 }
+
+/**
+ * Filters 'img' elements in post content to add 'srcset' and 'sizes' attributes.
+ *
+ * @since 4.4.0
+ * @deprecated 5.5.0
+ *
+ * @see wp_image_add_srcset_and_sizes()
+ *
+ * @param string $content The raw post content to be filtered.
+ * @return string Converted content with 'srcset' and 'sizes' attributes added to images.
+ */
+function wp_make_content_images_responsive( $content ) {
+        _deprecated_function( __FUNCTION__, '5.5.0', 'wp_filter_content_tags()' );
+
+        // This will also add the `loading` attribute to `img` tags, if enabled.
+        return wp_filter_content_tags( $content );
+}

src/wp-includes/media.php

@@ -1021 +1021 @@
 function wp_get_attachment_image( $attachment_id, $size = 'thumbnail', $icon = false, $attr = '' ) {
         $html  = '';
         $image = wp_get_attachment_image_src( $attachment_id, $size, $icon );
+
         if ( $image ) {
                 list( $src, $width, $height ) = $image;
-                $hwstring                     = image_hwstring( $width, $height );
-                $size_class                   = $size;
+
+                $attachment = get_post( $attachment_id );
+                $hwstring   = image_hwstring( $width, $height );
+                $size_class = $size;
+
                 if ( is_array( $size_class ) ) {
                         $size_class = join( 'x', $size_class );
                 }
-                $attachment   = get_post( $attachment_id );
+
                 $default_attr = array(
                         'src'   => $src,
                         'class' => "attachment-$size_class size-$size_class",
@@ -1035 +1039 @@
                         'alt'   => trim( strip_tags( get_post_meta( $attachment_id, '_wp_attachment_image_alt', true ) ) ),
                 );
 
+                // Add `loading` attribute.
+                if ( wp_lazy_loading_enabled( 'img', 'wp_get_attachment_image' ) ) {
+                        $default_attr['loading'] = 'lazy';
+                }
+
                 $attr = wp_parse_args( $attr, $default_attr );
 
                 // Generate 'srcset' and 'sizes' if not already present.
@@ -1061 +1070 @@
                  *
                  * @since 2.8.0
                  *
-                 * @param string[]     $attr       Array of attribute values for the image markup, keyed by attribute name.
+                 * @param array        $attr       Array of attribute values for the image markup, keyed by attribute name.
                  *                                 See wp_get_attachment_image().
                  * @param WP_Post      $attachment Image attachment post.
                  * @param string|array $size       Requested size. Image size or array of width and height values
@@ -1068 +1077 @@
                  *                                 (in that order). Default 'thumbnail'.
                  */
                 $attr = apply_filters( 'wp_get_attachment_image_attributes', $attr, $attachment, $size );
+
                 $attr = array_map( 'esc_attr', $attr );
                 $html = rtrim( "<img $hwstring" );
+
                 foreach ( $attr as $name => $value ) {
                         $html .= " $name=" . '"' . $value . '"';
                 }
+
                 $html .= ' />';
         }
 
@@ -1478 +1490 @@
 }
 
 /**
- * Filters 'img' elements in post content to add 'srcset' and 'sizes' attributes.
- *
- * @since 4.4.0
- *
- * @see wp_image_add_srcset_and_sizes()
- *
- * @param string $content The raw post content to be filtered.
- * @return string Converted content with 'srcset' and 'sizes' attributes added to images.
- */
-function wp_make_content_images_responsive( $content ) {
-        if ( ! preg_match_all( '/<img [^>]+>/', $content, $matches ) ) {
-                return $content;
-        }
-
-        $selected_images = array();
-        $attachment_ids  = array();
-
-        foreach ( $matches[0] as $image ) {
-                if ( false === strpos( $image, ' srcset=' ) && preg_match( '/wp-image-([0-9]+)/i', $image, $class_id ) ) {
-                        $attachment_id = absint( $class_id[1] );
-
-                        if ( $attachment_id ) {
-                                /*
-                                 * If exactly the same image tag is used more than once, overwrite it.
-                                 * All identical tags will be replaced later with 'str_replace()'.
-                                 */
-                                $selected_images[ $image ] = $attachment_id;
-                                // Overwrite the ID when the same image is included more than once.
-                                $attachment_ids[ $attachment_id ] = true;
-                        }
-                }
-        }
-
-        if ( count( $attachment_ids ) > 1 ) {
-                /*
-                 * Warm the object cache with post and meta information for all found
-                 * images to avoid making individual database calls.
-                 */
-                _prime_post_caches( array_keys( $attachment_ids ), false, true );
-        }
-
-        foreach ( $selected_images as $image => $attachment_id ) {
-                $image_meta = wp_get_attachment_metadata( $attachment_id );
-                $content    = str_replace( $image, wp_image_add_srcset_and_sizes( $image, $image_meta, $attachment_id ), $content );
-        }
-
-        return $content;
-}
-
-/**
  * Adds 'srcset' and 'sizes' attributes to an existing 'img' element.
  *
  * @since 4.4.0
@@ -1617 +1579 @@
 }
 
 /**
+ * Determine whether to add the `loading` attribute to the specified tag in the specified context.
+ *
+ * @since 5.5.0
+ *
+ * @param string $tag_name The tag name.
+ * @param string $context Additional context, like the current filter name or the function name from where this was called.
+ * @return bool Whether to add the attribute.
+ */
+function wp_lazy_loading_enabled( $tag_name, $context ) {
+        // By default add to all 'img' tags.
+        // See https://html.spec.whatwg.org/multipage/embedded-content.html#attr-img-loading
+        $default = ( 'img' === $tag_name );
+
+        /**
+         * Filters whether to add the `loading` attribute to the specified tag in the specified context.
+         *
+         * @since 5.5.0
+         *
+         * @param bool   $default Default value.
+         * @param string $tag_name The tag name.
+         * @param string $context Additional context, like the current filter name or the function name from where this was called.
+         */
+        return (bool) apply_filters( 'wp_lazy_loading_enabled', $default, $tag_name, $context );
+}
+
+/**
+ * Filters specific tags in post content and modifies their markup.
+ *
+ * This function adds `srcset`, `sizes`, and `loading` attributes to `img` HTML tags.
+ *
+ * @since 5.5.0
+ *
+ * @see wp_img_tag_add_loading_attr()
+ * @see wp_img_tag_add_srcset_and_sizes_attr()
+ *
+ * @param string $content The HTML content to be filtered.
+ * @param string $context Optional. Additional context to pass to the filters. Defaults to `current_filter()` when not set.
+ * @return string Converted content with images modified.
+ */
+function wp_filter_content_tags( $content, $context = null ) {
+        if ( null === $context ) {
+                $context = current_filter();
+        }
+
+        $add_loading_attr = wp_lazy_loading_enabled( 'img', $context );
+
+        if ( false === strpos( $content, '<img' ) ) {
+                return $content;
+        }
+
+        if ( ! preg_match_all( '/<img\s[^>]+>/', $content, $matches ) ) {
+                return $content;
+        }
+
+        // List of the unique `img` tags found in $content.
+        $images = array();
+
+        foreach ( $matches[0] as $image ) {
+                if ( preg_match( '/wp-image-([0-9]+)/i', $image, $class_id ) ) {
+                        $attachment_id = absint( $class_id[1] );
+
+                        if ( $attachment_id ) {
+                                // If exactly the same image tag is used more than once, overwrite it.
+                                // All identical tags will be replaced later with 'str_replace()'.
+                                $images[ $image ] = $attachment_id;
+                                continue;
+                        }
+                }
+
+                $images[ $image ] = 0;
+        }
+
+        // Reduce the array to unique attachment IDs.
+        $attachment_ids = array_unique( array_filter( array_values( $images ) ) );
+
+        if ( count( $attachment_ids ) > 1 ) {
+                /*
+                 * Warm the object cache with post and meta information for all found
+                 * images to avoid making individual database calls.
+                 */
+                _prime_post_caches( $attachment_ids, false, true );
+        }
+
+        foreach ( $images as $image => $attachment_id ) {
+                $filtered_image = $image;
+
+                // Add 'srcset' and 'sizes' attributes if applicable.
+                if ( $attachment_id > 0 && false === strpos( $filtered_image, ' srcset=' ) ) {
+                        $filtered_image = wp_img_tag_add_srcset_and_sizes_attr( $filtered_image, $context, $attachment_id );
+                }
+
+                // Add 'loading' attribute if applicable.
+                if ( $add_loading_attr && false === strpos( $filtered_image, ' loading=' ) ) {
+                        $filtered_image = wp_img_tag_add_loading_attr( $filtered_image, $context );
+                }
+
+                if ( $filtered_image !== $image ) {
+                        $content = str_replace( $image, $filtered_image, $content );
+                }
+        }
+
+        return $content;
+}
+
+/**
+ * Adds `loading` attribute to an existing `img` HTML tag.
+ *
+ * @since 5.5.0
+ *
+ * @param string $image   The HTML `img` tag where the attribute should be added.
+ * @param string $context Additional context to pass to the filters.
+ * @return string Converted `img` tag with `loading` attribute added.
+ */
+function wp_img_tag_add_loading_attr( $image, $context ) {
+        /**
+         * Filters the `loading` attribute value. Default `lazy`.
+         *
+         * Returning `false` or an empty string will not add the attribute.
+         * Returning `true` will add the default value.
+         *
+         * @since 5.5.0
+         *
+         * @param string $value   The 'loading' attribute value, defaults to `lazy`.
+         * @param string $image   The HTML 'img' element to be filtered.
+         * @param string $context Additional context about how the function was called or where the img tag is.
+         */
+        $value = apply_filters( 'wp_img_tag_add_loading_attr', 'lazy', $image, $context );
+
+        if ( $value ) {
+                if ( ! in_array( $value, array( 'lazy', 'eager' ), true ) ) {
+                        $value = 'lazy';
+                }
+
+                $quote = null;
+
+                // Get the quote character used in the tag.
+                // Normally it will be a double quote but in some rare cases may be a single quote.
+                // This also checks if the `img` tag is not malformed, i.e. it has a `src` attribute.
+                if ( false !== strpos( $image, ' src="' ) ) {
+                        $quote = '"';
+                } elseif ( preg_match( '/\ssrc\s*=(["\'])/', $image, $matches ) ) {
+                        $quote = $matches[1];
+                }
+
+                if ( $quote ) {
+                        $loading = "loading={$quote}{$value}{$quote}";
+
+                        return str_replace( '<img', "<img {$loading}", $image );
+                }
+        }
+
+        return $image;
+}
+
+/**
+ * Adds `srcset` and `sizes` attributes to an existing `img` HTML tag.
+ *
+ * @since 5.5.0
+ *
+ * @param string $image         The HTML `img` tag where the attribute should be added.
+ * @param string $context       Additional context to pass to the filters.
+ * @param int    $attachment_id Image attachment ID.
+ * @return string Converted 'img' element with 'loading' attribute added.
+ */
+function wp_img_tag_add_srcset_and_sizes_attr( $image, $context, $attachment_id ) {
+        /**
+         * Filters whether to add the `srcset` and `sizes` HTML attributes to the img tag. Default `true`.
+         *
+         * Returning anything else than `true` will not add the attributes.
+         *
+         * @since 5.5.0
+         *
+         * @param bool   $value         The filtered value, defaults to `true`.
+         * @param string $image         The HTML `img` tag where the attribute should be added.
+         * @param string $context       Additional context about how the function was called or where the img tag is.
+         * @param int    $attachment_id The image attachment ID.
+         */
+        $add = apply_filters( 'wp_img_tag_add_srcset_and_sizes_attr', true, $image, $context, $attachment_id );
+
+        if ( true === $add ) {
+                $image_meta = wp_get_attachment_metadata( $attachment_id );
+                return wp_image_add_srcset_and_sizes( $image, $image_meta, $attachment_id );
+        }
+
+        return $image;
+}
+
+/**
  * Adds a 'wp-post-image' class to post thumbnails. Internal use only.
  *
  * Uses the {@see 'begin_fetch_post_thumbnail_html'} and {@see 'end_fetch_post_thumbnail_html'}

src/wp-includes/pluggable.php

@@ -2601 +2601 @@
          *                                       Default null.
          *     @type bool         $force_display Whether to always show the avatar - ignores the show_avatars option.
          *                                       Default false.
+         *     @type string       $loading       Value for the `loading` attribute.
+         *                                       Default null.
          *     @type string       $extra_attr    HTML attributes to insert in the IMG element. Is not sanitized. Default empty.
          * }
          * @return string|false `<img>` tag for the user's avatar. False on failure.
@@ -2618 +2620 @@
                         'alt'           => '',
                         'class'         => null,
                         'force_display' => false,
+                        'loading'       => null,
                         'extra_attr'    => '',
                 );
 
+                if ( wp_lazy_loading_enabled( 'img', 'get_avatar' ) ) {
+                        $defaults['loading'] = 'lazy';
+                }
+
                 if ( empty( $args ) ) {
                         $args = array();
                 }
@@ -2690 +2697 @@
                         }
                 }
 
+                // Add `loading` attribute.
+                $extra_attr = $args['extra_attr'];
+                $loading    = $args['loading'];
+
+                if ( in_array( $loading, array( 'lazy', 'eager' ), true ) && ! preg_match( '/\bloading\s*=/', $extra_attr ) ) {
+                        if ( ! empty( $extra_attr ) ) {
+                                $extra_attr .= ' ';
+                        }
+
+                        $extra_attr .= "loading='{$loading}'";
+                }
+
                 $avatar = sprintf(
                         "<img alt='%s' src='%s' srcset='%s' class='%s' height='%d' width='%d' %s/>",
                         esc_attr( $args['alt'] ),
@@ -2698 +2717 @@
                         esc_attr( join( ' ', $class ) ),
                         (int) $args['height'],
                         (int) $args['width'],
-                        $args['extra_attr']
+                        $extra_attr
                 );
 
                 /**

tests/phpunit/tests/avatar.php

@@ -169 +169 @@
 
         public function test_get_avatar() {
                 $img = get_avatar( 1 );
-                $this->assertEquals( preg_match( "|^<img alt='[^']*' src='[^']*' srcset='[^']*' class='[^']*' height='[^']*' width='[^']*' />$|", $img ), 1 );
+                $this->assertEquals( preg_match( "|^<img alt='[^']*' src='[^']*' srcset='[^']*' class='[^']*' height='[^']*' width='[^']*' loading='lazy'/>$|", $img ), 1 );
         }
 
         public function test_get_avatar_size() {

tests/phpunit/tests/media.php

@@ -1332 +1332 @@
          */
         function test_wp_get_attachment_image_defaults() {
                 $image    = image_downsize( self::$large_id, 'thumbnail' );
-                $expected = sprintf( '<img width="%1$d" height="%2$d" src="%3$s" class="attachment-thumbnail size-thumbnail" alt="" />', $image[1], $image[2], $image[0] );
+                $expected = sprintf( '<img width="%1$d" height="%2$d" src="%3$s" class="attachment-thumbnail size-thumbnail" alt="" loading="lazy" />', $image[1], $image[2], $image[0] );
 
                 $this->assertEquals( $expected, wp_get_attachment_image( self::$large_id ) );
         }
@@ -1347 +1347 @@
                 update_post_meta( self::$large_id, '_wp_attachment_image_alt', 'Some very clever alt text', true );
 
                 $image    = image_downsize( self::$large_id, 'thumbnail' );
-                $expected = sprintf( '<img width="%1$d" height="%2$d" src="%3$s" class="attachment-thumbnail size-thumbnail" alt="Some very clever alt text" />', $image[1], $image[2], $image[0] );
+                $expected = sprintf( '<img width="%1$d" height="%2$d" src="%3$s" class="attachment-thumbnail size-thumbnail" alt="Some very clever alt text" loading="lazy" />', $image[1], $image[2], $image[0] );
 
                 $this->assertEquals( $expected, wp_get_attachment_image( self::$large_id ) );
 
@@ -1964 +1964 @@
         /**
          * @ticket 33641
          */
-        function test_wp_make_content_images_responsive() {
+        function test_wp_filter_content_tags() {
                 $image_meta = wp_get_attachment_metadata( self::$large_id );
                 $size_array = $this->_get_image_size_array_from_meta( $image_meta, 'medium' );
 
@@ -1973 +1973 @@
 
                 // Function used to build HTML for the editor.
                 $img                  = get_image_tag( self::$large_id, '', '', '', 'medium' );
+                $img                  = wp_img_tag_add_loading_attr( $img, 'test' );
                 $img_no_size_in_class = str_replace( 'size-', '', $img );
                 $img_no_width_height  = str_replace( ' width="' . $size_array[0] . '"', '', $img );
                 $img_no_width_height  = str_replace( ' height="' . $size_array[1] . '"', '', $img_no_width_height );
@@ -2014 +2015 @@
                 $content_unfiltered = sprintf( $content, $img, $img_no_size_in_class, $img_no_width_height, $img_no_size_id, $img_with_sizes_attr, $img_xhtml, $img_html5 );
                 $content_filtered   = sprintf( $content, $respimg, $respimg_no_size_in_class, $respimg_no_width_height, $img_no_size_id, $respimg_with_sizes_attr, $respimg_xhtml, $respimg_html5 );
 
-                $this->assertSame( $content_filtered, wp_make_content_images_responsive( $content_unfiltered ) );
+                $this->assertSame( $content_filtered, wp_filter_content_tags( $content_unfiltered ) );
         }
 
         /**
@@ -2028 +2029 @@
          * @ticket 34898
          * @ticket 33641
          */
-        function test_wp_make_content_images_responsive_wrong() {
-                $image = get_image_tag( self::$large_id, '', '', '', 'medium' );
+        function test_wp_filter_content_tags_wrong() {
+                $img = get_image_tag( self::$large_id, '', '', '', 'medium' );
+                $img = wp_img_tag_add_loading_attr( $img, 'test' );
 
                 // Replace the src URL.
-                $image_wrong_src = preg_replace( '|src="[^"]+"|', 'src="http://' . WP_TESTS_DOMAIN . '/wp-content/uploads/foo.jpg"', $image );
+                $image_wrong_src = preg_replace( '|src="[^"]+"|', 'src="http://' . WP_TESTS_DOMAIN . '/wp-content/uploads/foo.jpg"', $img );
 
-                $this->assertSame( $image_wrong_src, wp_make_content_images_responsive( $image_wrong_src ) );
+                $this->assertSame( $image_wrong_src, wp_filter_content_tags( $image_wrong_src ) );
         }
 
         /**
          * @ticket 33641
          */
-        function test_wp_make_content_images_responsive_with_preexisting_srcset() {
+        function test_wp_filter_content_tags_with_preexisting_srcset() {
                 // Generate HTML and add a dummy srcset attribute.
-                $image_html = get_image_tag( self::$large_id, '', '', '', 'medium' );
-                $image_html = preg_replace( '|<img ([^>]+) />|', '<img $1 ' . 'srcset="image2x.jpg 2x" />', $image_html );
+                $img = get_image_tag( self::$large_id, '', '', '', 'medium' );
+                $img = wp_img_tag_add_loading_attr( $img, 'test' );
+                $img = preg_replace( '|<img ([^>]+) />|', '<img $1 ' . 'srcset="image2x.jpg 2x" />', $img );
 
                 // The content filter should return the image unchanged.
-                $this->assertSame( $image_html, wp_make_content_images_responsive( $image_html ) );
+                $this->assertSame( $img, wp_filter_content_tags( $img ) );
         }
 
         /**
@@ -2097 +2100 @@
          * @ticket 35045
          * @ticket 33641
          */
-        function test_wp_make_content_images_responsive_schemes() {
+        function test_wp_filter_content_tags_schemes() {
                 $image_meta = wp_get_attachment_metadata( self::$large_id );
                 $size_array = $this->_get_image_size_array_from_meta( $image_meta, 'medium' );
 
@@ -2106 +2109 @@
 
                 // Build HTML for the editor.
                 $img          = get_image_tag( self::$large_id, '', '', '', 'medium' );
+                $img          = wp_img_tag_add_loading_attr( $img, 'test' );
                 $img_https    = str_replace( 'http://', 'https://', $img );
                 $img_relative = str_replace( 'http://', '//', $img );
 
@@ -2126 +2130 @@
 
                 $unfiltered = sprintf( $content, $img, $img_https, $img_relative );
                 $expected   = sprintf( $content, $respimg, $respimg_https, $respimg_relative );
-                $actual     = wp_make_content_images_responsive( $unfiltered );
+                $actual     = wp_filter_content_tags( $unfiltered );
 
                 $this->assertSame( $expected, $actual );
         }
@@ -2260 +2264 @@
                 $month  = gmdate( 'm' );
 
                 $expected = '<img width="999" height="999" src="http://' . WP_TESTS_DOMAIN . '/wp-content/uploads/' . $year . '/' . $month . '/test-image-testsize-999x999.png"' .
-                        ' class="attachment-testsize size-testsize" alt=""' .
+                        ' class="attachment-testsize size-testsize" alt="" loading="lazy"' .
                         ' srcset="http://' . WP_TESTS_DOMAIN . '/wp-content/uploads/' . $year . '/' . $month . '/test-image-testsize-999x999.png 999w,' .
                                 ' http://' . WP_TESTS_DOMAIN . '/wp-content/uploads/' . $year . '/' . $month . '/test-image-large-150x150.png 150w"' .
                                 ' sizes="(max-width: 999px) 100vw, 999px" />';
@@ -2523 +2527 @@
 
                 $this->assertSame( $expected, $url );
         }
+
+        /**
+         * @ticket 44427
+         */
+        function test_wp_lazy_load_content_media() {
+                $img       = get_image_tag( self::$large_id, '', '', '', 'medium' );
+                $img_xhtml = str_replace( ' />', '/>', $img );
+                $img_html5 = str_replace( ' />', '>', $img );
+                $iframe    = '<iframe src="https://www.example.com"></iframe>';
+
+                $lazy_img       = wp_img_tag_add_loading_attr( $img, 'test' );
+                $lazy_img_xhtml = wp_img_tag_add_loading_attr( $img_xhtml, 'test' );
+                $lazy_img_html5 = wp_img_tag_add_loading_attr( $img_html5, 'test' );
+
+                // The following should not be modified because there already is a 'loading' attribute.
+                $img_eager = str_replace( ' />', ' loading="eager" />', $img );
+
+                $content = '
+                        <p>Image, standard.</p>
+                        %1$s
+                        <p>Image, XHTML 1.0 style (no space before the closing slash).</p>
+                        %2$s
+                        <p>Image, HTML 5.0 style.</p>
+                        %3$s
+                        <p>Image, with pre-existing "loading" attribute.</p>
+                        %5$s
+                        <p>Iframe, standard. Should not be modified.</p>
+                        %4$s';
+
+                $content_unfiltered = sprintf( $content, $img, $img_xhtml, $img_html5, $iframe, $img_eager );
+                $content_filtered   = sprintf( $content, $lazy_img, $lazy_img_xhtml, $lazy_img_html5, $iframe, $img_eager );
+
+                // Do not add srcset and sizes.
+                add_filter( 'wp_img_tag_add_srcset_and_sizes_attr', '__return_false' );
+
+                $this->assertSame( $content_filtered, wp_filter_content_tags( $content_unfiltered ) );
+
+                remove_filter( 'wp_img_tag_add_srcset_and_sizes_attr', '__return_false' );
+        }
+
+        /**
+         * @ticket 44427
+         */
+        function test_wp_lazy_load_content_media_opted_in() {
+                $img      = get_image_tag( self::$large_id, '', '', '', 'medium' );
+                $lazy_img = wp_img_tag_add_loading_attr( $img, 'test' );
+
+                $content = '
+                        <p>Image, standard.</p>
+                        %1$s';
+
+                $content_unfiltered = sprintf( $content, $img );
+                $content_filtered   = sprintf( $content, $lazy_img );
+
+                // Do not add srcset and sizes while testing.
+                add_filter( 'wp_img_tag_add_srcset_and_sizes_attr', '__return_false' );
+
+                // Enable globally for all tags.
+                add_filter( 'wp_lazy_loading_enabled', '__return_true' );
+
+                $this->assertSame( $content_filtered, wp_filter_content_tags( $content_unfiltered ) );
+                remove_filter( 'wp_lazy_loading_enabled', '__return_true' );
+                remove_filter( 'wp_img_tag_add_srcset_and_sizes_attr', '__return_false' );
+        }
+
+        /**
+         * @ticket 44427
+         */
+        function test_wp_lazy_load_content_media_opted_out() {
+                $img = get_image_tag( self::$large_id, '', '', '', 'medium' );
+
+                $content = '
+                        <p>Image, standard.</p>
+                        %1$s';
+                $content = sprintf( $content, $img );
+
+                // Do not add srcset and sizes while testing.
+                add_filter( 'wp_img_tag_add_srcset_and_sizes_attr', '__return_false' );
+
+                // Disable globally for all tags.
+                add_filter( 'wp_lazy_loading_enabled', '__return_false' );
+
+                $this->assertSame( $content, wp_filter_content_tags( $content ) );
+                remove_filter( 'wp_lazy_loading_enabled', '__return_false' );
+                remove_filter( 'wp_img_tag_add_srcset_and_sizes_attr', '__return_false' );
+        }
+
+        /**
+         * @ticket 44427
+         */
+        function test_wp_img_tag_add_loading_attr_single_quote() {
+                $img = "<img src='example.png' alt='' width='300' height='225' />";
+                $img = wp_img_tag_add_loading_attr( $img, 'test' );
+
+                $this->assertContains( " loading='lazy'", $img );
+        }
 }
 
 /**