diff --git a/src/wp-includes/default-filters.php b/src/wp-includes/default-filters.php
index 38655c5f7d..b18dbe7544 100644
--- a/src/wp-includes/default-filters.php
+++ b/src/wp-includes/default-filters.php
@@ -175,13 +175,14 @@ add_filter( 'the_content', 'convert_smilies', 20 );
 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' );
 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' );
@@ -207,6 +208,7 @@ add_filter( 'widget_text_content', 'wptexturize' );
 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.
diff --git a/src/wp-includes/deprecated.php b/src/wp-includes/deprecated.php
index 7804c8efcb..625dee4ac5 100644
--- a/src/wp-includes/deprecated.php
+++ b/src/wp-includes/deprecated.php
@@ -2509,11 +2509,11 @@ function is_term( $term, $taxonomy = '', $parent = 0 ) {
  * 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
  *
@@ -3716,11 +3716,11 @@ function get_comments_popup_template() {
 
 /**
  * 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
  *
@@ -3950,3 +3950,21 @@ function wp_ajax_press_this_add_category() {
 		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 );
+}
diff --git a/src/wp-includes/media.php b/src/wp-includes/media.php
index b5843e6af8..d1dfbc24a5 100644
--- a/src/wp-includes/media.php
+++ b/src/wp-includes/media.php
@@ -1021,20 +1021,29 @@ function wp_get_attachment_image_src( $attachment_id, $size = 'thumbnail', $icon
 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",
 			'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.
@@ -1061,18 +1070,21 @@ function wp_get_attachment_image( $attachment_id, $size = 'thumbnail', $icon = f
 		 *
 		 * @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
 		 *                                 (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 .= ' />';
 	}
 
@@ -1477,56 +1489,6 @@ function wp_calculate_image_sizes( $size, $image_src = null, $image_meta = null,
 	return apply_filters( 'wp_calculate_image_sizes', $sizes, $size, $image_src, $image_meta, $attachment_id );
 }
 
-/**
- * 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.
  *
@@ -1616,6 +1578,193 @@ function wp_image_add_srcset_and_sizes( $image, $image_meta, $attachment_id ) {
 	return $image;
 }
 
+/**
+ * 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 `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` tag 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;
+
+		// Check if the img tag is valid (has `src` attribute) and get the quote character.
+		// In almost all cases it will have src and a double quote.
+		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.
  *
diff --git a/src/wp-includes/pluggable.php b/src/wp-includes/pluggable.php
index 69543e88c1..c5899c6c36 100644
--- a/src/wp-includes/pluggable.php
+++ b/src/wp-includes/pluggable.php
@@ -2606,6 +2606,8 @@ if ( ! function_exists( 'get_avatar' ) ) :
 	 *                                       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.
@@ -2623,9 +2625,14 @@ if ( ! function_exists( 'get_avatar' ) ) :
 			'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();
 		}
@@ -2695,6 +2702,18 @@ if ( ! function_exists( 'get_avatar' ) ) :
 			}
 		}
 
+		// 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'] ),
@@ -2703,7 +2722,7 @@ if ( ! function_exists( 'get_avatar' ) ) :
 			esc_attr( join( ' ', $class ) ),
 			(int) $args['height'],
 			(int) $args['width'],
-			$args['extra_attr']
+			$extra_attr
 		);
 
 		/**
diff --git a/tests/phpunit/tests/avatar.php b/tests/phpunit/tests/avatar.php
index 898c30f296..5649944230 100644
--- a/tests/phpunit/tests/avatar.php
+++ b/tests/phpunit/tests/avatar.php
@@ -169,7 +169,7 @@ class Tests_Avatar extends WP_UnitTestCase {
 
 	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() {
diff --git a/tests/phpunit/tests/media.php b/tests/phpunit/tests/media.php
index e9dbd27db5..3621711749 100644
--- a/tests/phpunit/tests/media.php
+++ b/tests/phpunit/tests/media.php
@@ -1332,7 +1332,7 @@ EOF;
 	 */
 	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 ) );
 	}
@@ -1347,7 +1347,7 @@ EOF;
 		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 ) );
 
@@ -1964,7 +1964,7 @@ EOF;
 	/**
 	 * @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' );
 
@@ -1973,6 +1973,7 @@ EOF;
 
 		// 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 );
@@ -2014,7 +2015,7 @@ EOF;
 		$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 ) );
 	}
 
 	/**
@@ -2028,25 +2029,27 @@ EOF;
 	 * @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 ) );
 	}
 
 	/**
@@ -2097,7 +2100,7 @@ EOF;
 	 * @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' );
 
@@ -2106,6 +2109,7 @@ EOF;
 
 		// 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 );
 
@@ -2126,7 +2130,7 @@ EOF;
 
 		$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 );
 	}
@@ -2260,7 +2264,7 @@ EOF;
 		$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" />';
@@ -2523,6 +2527,102 @@ EOF;
 
 		$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 );
+	}
 }
 
 /**