diff --git a/tests/phpunit/tests/media.php b/tests/phpunit/tests/media.php
index b9b98dd..98b7c67 100644
--- a/tests/phpunit/tests/media.php
+++ b/tests/phpunit/tests/media.php
@@ -52,7 +52,7 @@ function test_wp_lazy_load_content_media() {
 		$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 );
 
-		$this->assertSame( $content_filtered, wp_add_lazy_load_attributes( $content_unfiltered ) );
+		$this->assertSame( $content_filtered, _wp_filter_html_tags( $content_unfiltered ) );
 	}
 
 	/**
@@ -75,10 +75,22 @@ function test_wp_lazy_load_content_media_opted_in() {
 		$content_unfiltered = sprintf( $content, $img, $iframe );
 		$content_filtered   = sprintf( $content, $lazy_img, $lazy_iframe );
 
-		add_filter( 'wp_get_lazy_load_tags', '__return_true' );
+		add_filter( 'wp_add_lazy_loading_to', '__return_true' );
+		add_filter( 'wp_get_tags_to_filter', array( $this, 'wp_get_tags_to_filter_callback' ) );
+		add_filter( 'wp_filter_iframe_tags', array( $this, 'wp_filter_iframe_tags_callback' ) );
 
-		$this->assertSame( $content_filtered, wp_add_lazy_load_attributes( $content_unfiltered ) );
-		remove_filter( 'wp_get_lazy_load_tags', '__return_true' );
+		$this->assertSame( $content_filtered, _wp_filter_html_tags( $content_unfiltered ) );
+		remove_filter( 'wp_add_lazy_loading_to', '__return_true' );
+		remove_filter( 'wp_get_tags_to_filter', array( $this, 'wp_get_tags_to_filter_callback' ) );
+		remove_filter( 'wp_filter_iframe_tags', array( $this, 'wp_filter_iframe_tags_callback' ) );
+	}
+
+	function wp_get_tags_to_filter_callback() {
+		return array( 'img', 'iframe' );
+	}
+
+	function wp_filter_iframe_tags_callback( $tag_html ) {
+		return str_replace( '<iframe', '<iframe loading="lazy"', $tag_html );
 	}
 
 	/**
@@ -92,9 +104,9 @@ function test_wp_lazy_load_content_media_opted_out() {
 			%1$s';
 		$content = sprintf( $content, $img );
 
-		add_filter( 'wp_get_lazy_load_tags', '__return_false' );
+		add_filter( 'wp_add_lazy_loading_to', '__return_false' );
 
-		$this->assertSame( $content, wp_add_lazy_load_attributes( $content ) );
-		remove_filter( 'wp_get_lazy_load_tags', '__return_false' );
+		$this->assertSame( $content, _wp_filter_html_tags( $content ) );
+		remove_filter( 'wp_add_lazy_loading_to', '__return_false' );
 	}
 }
diff --git a/wp-lazy-loading.php b/wp-lazy-loading.php
index 2668eb3..a96961e 100644
--- a/wp-lazy-loading.php
+++ b/wp-lazy-loading.php
@@ -32,15 +32,30 @@
 function _wp_lazy_loading_initialize_filters() {
 	// The following filters would be merged into core.
 	foreach ( array( 'the_content', 'the_excerpt', 'comment_text', 'widget_text_content' ) as $filter ) {
-		// Before parsing blocks and shortcodes.
-		// TODO: Comments do not support images. Revisit.
-		// TODO: This should not exclude images from dynamic blocks and shortcodes. Look at fixing the filter priority.
-		add_filter( $filter, 'wp_add_lazy_load_attributes', 8 );
+		// After parsing blocks and shortcodes.
+		// TODO: Comments and excerpts do not support images. Revisit?
+		// TODO: This includes images added from shortcodes. Needs more testing.
+		add_filter( $filter, '_wp_filter_html_tags', 25 );
 	}
 
+	add_filter( 'wp_filter_img_tags', 'wp_add_lazy_loading_to_img_tags', 10, 3 );
+
 	// The following filters are only needed while this is a feature plugin.
 	add_filter( 'wp_get_attachment_image_attributes', '_wp_lazy_loading_add_attribute_to_attachment_image' );
 	add_filter( 'get_avatar', '_wp_lazy_loading_add_attribute_to_avatar' );
+
+	// Exprerimental, testing only.
+	// To test with iframes: add `?wp-lazy-loading-iframes` to the current URL.
+	if ( isset( $_GET['wp-lazy-loading-iframes'] ) ) {
+		// Add to all tags.
+		add_filter( 'wp_add_lazy_loading_to', '__return_true' );
+
+		// Add filterig for the iframe tag.
+		add_filter( 'wp_get_tags_to_filter', 'wp_lazy_loading_add_iframe_tag', 10, 2 );
+
+		// Add the actual attribute, and filter the tag after that.
+		add_filter( 'wp_filter_iframe_tags', 'wp_add_lazy_loading_to_iframe_tags', 10, 3 );
+	}
 }
 
 add_action( 'plugins_loaded', '_wp_lazy_loading_initialize_filters', 1 );
@@ -62,7 +77,7 @@ function _wp_lazy_loading_initialize_filters() {
  * @return string Modified tag.
  */
 function _wp_lazy_loading_add_attribute_to_avatar( $avatar ) {
-	if ( in_array( 'img', wp_get_lazy_load_tags(), true ) && false === strpos( $avatar, ' loading=' ) ) {
+	if ( wp_add_lazy_loading_to( 'img', 'get_avatar' ) && false === strpos( $avatar, ' loading=' ) ) {
 		$avatar = str_replace( '<img ', '<img loading="lazy" ', $avatar );
 	}
 
@@ -83,7 +98,7 @@ function _wp_lazy_loading_add_attribute_to_avatar( $avatar ) {
  * @return array Modified attributes.
  */
 function _wp_lazy_loading_add_attribute_to_attachment_image( $attr ) {
-	if ( in_array( 'img', wp_get_lazy_load_tags(), true ) && ! isset( $attr['loading'] ) ) {
+	if ( wp_add_lazy_loading_to( 'img', 'wp_get_attachment_image' ) && ! isset( $attr['loading'] ) ) {
 		$attr['loading'] = 'lazy';
 	}
 
@@ -91,73 +106,172 @@ function _wp_lazy_loading_add_attribute_to_attachment_image( $attr ) {
 }
 
 
+// Helper functions to enable adding of `loading="lazy"` to iframes (not for merging for now).
+
+function wp_add_lazy_loading_to_iframe_tags( $tag_html, $content, $context = null ) {
+	if ( wp_add_lazy_loading_to( 'iframe', $context ) && ! preg_match( '/\bloading\s*=/', $tag_html ) ) {
+		$unfiltered = $tag_html;
+		$tag_html   = str_replace( '<iframe', '<iframe loading="lazy"', $tag_html );
+
+		// See the docs for the similar 'wp_add_lazy_loading_to_img_tags'.
+		$tag_html = apply_filters( 'wp_add_lazy_loading_to_iframe_tags', $tag_html, $unfiltered, $content, $context );
+	}
+
+	return $tag_html;
+}
+
+function wp_lazy_loading_add_iframe_tag( $default_tags, $context ) {
+	if ( 'the_content' === $context ) {
+		$default_tags[] = 'iframe';
+	}
+
+	return $default_tags;
+}
+
+
 // The following functions will be merged to core.
 
+/**
+ * Determine whether to add the `loading` attribute to the specified tag in the specified context.
+ *
+ * @since (TBD)
+ *
+ * @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 boolean Whether to add the attribute.
+ */
+function wp_add_lazy_loading_to( $tag_name, $context ) {
+	// By default add to all 'img' tags.
+	// See https://github.com/whatwg/html/issues/2806
+	$add = ( 'img' === $tag_name );
+
+	/**
+	 * Filters whether to add the `loading` attribute to the specified tag in the specified context.
+	 *
+	 * @since (TBD)
+	 *
+	 * @param boolean $add Defatls 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_add_lazy_loading_to', $add, $tag_name, $context );
+}
+
 /**
  * Get the HTML tags to lazy-load.
  *
  * @since (TBD)
  *
- * @return array List of tags to add loading="lazy" attributes to.
+ * @param string $context Context for use when filtering the list of tag names. For example `current_filter()`.
+ * @return array List of tags to filter on display.
  */
-function wp_get_lazy_load_tags() {
-	// See https://github.com/whatwg/html/issues/2806.
-	$supported_tags = array( 'img', 'iframe' );
+function wp_get_tags_to_filter( $context ) {
+	// For adding the `loading` attribute. See https://github.com/whatwg/html/issues/2806.
+	$default_tags = array( 'img' );
 
 	/**
-	 * Filters on which HTML tags to add `loading="lazy"`.
+	 * Add or remove HTML tags that will be filtered on display.
 	 *
 	 * @since (TBD)
 	 *
-	 * @param array $tags List of tags to add loading="lazy" attributes to. Default is an array with 'img'.
+	 * @param array $tags List of tags to filter. Default is an array with 'img'.
 	 *                    Supports passing a boolean: `false` removes all tags, `true` resets the tags to the default value.
-	 *                    Example: `add_filter( 'wp_get_lazy_load_tags', '__return_true' );` will enable it for images and iframes.
-	 *                    Note that the HTML specs are not finalized yet. See https://github.com/whatwg/html/pull/3752/files,
-	 *                    and support for iframes has been postponed (for now).
+	 *                    Example: `add_filter( 'wp_get_tags_to_filter', '__return_true' );` will enable filtering of img tags.
 	 */
-	$tags = apply_filters( 'wp_get_lazy_load_tags', array( 'img' ) );
+	$tags = apply_filters( 'wp_get_tags_to_filter', $default_tags, $context );
 
 	// Support a boolean, false to disable, true to reset to default.
 	if ( ! is_array( $tags ) ) {
-		return $tags === true ? $supported_tags : array();
+		return $tags === true ? $default_tags : array();
 	}
 
-	// Only include supported tags.
-	return array_values( array_intersect( $tags, $supported_tags ) );
+	return array_unique( $tags );
 }
 
 // TODO: update docs.
 /**
- * Add `loading="lazy"` to `img` and/or `iframe` HTML tags.
+ * Add `loading="lazy"` to `<img>` tags.
  *
  * Currently the "loading" attribute is only supported for `img`, and is enabled by default.
- * Support for `iframe` is for testing purposes only.
+ * Support for `iframe` can be enabled for testing purposes.
  *
  * @since (TBD)
  *
- * @param string $content The raw post content to be filtered.
+ * @param string $tag_html The tag markup.
+ * @param string $content The (HTML) content where the img tag is.
+ * @param string $context Optional. Additional context passed to the function.
  * @return string Converted content with 'loading' attributes added to images.
  */
-function wp_add_lazy_load_attributes( $content ) {
-	$tags = wp_get_lazy_load_tags();
+function wp_add_lazy_loading_to_img_tags( $tag_html, $content, $context = null ) {
+	if ( wp_add_lazy_loading_to( 'img', $context ) && ! preg_match( '/\bloading\s*=/', $tag_html ) ) {
+		$unfiltered = $tag_html;
+		$tag_html   = str_replace( '<img', '<img loading="lazy"', $tag_html );
+
+		/**
+		 * Filters on adding `loading="lazy"` to img tags.
+		 *
+		 * @since (TBD)
+		 *
+		 * @param string $tag_html The img tag with added attribute.
+		 * @param string $unfiltered The img tag before adding the attribute.
+		 * @param string $content The (HTML) content where the img tag is.
+		 * @param string $context Optional. Additional context passed to the function.
+		 */
+		$tag_html = apply_filters( 'wp_add_lazy_loading_to_img_tags', $tag_html, $unfiltered, $content, $context );
+	}
+
+	return $tag_html;
+}
+
+/**
+ * Find specific HTML tags in the passed content and filter them.
+ *
+ * @since (TBD)
+ * @access private
+ *
+ * @param string $content The (HTML) content to be searched.
+ * @param string $context Optional. Additional context that may be passed when calling the function directly.
+ * @return string The content with filtered tags.
+ */
+function _wp_filter_html_tags( $content, $context = null ) {
+	if ( null === $context ) {
+		$context = current_filter();
+	}
+
+	$tags = wp_get_tags_to_filter( $context );
+
+	// Check if the tag exists in $content and there are filters for the tag.
+	foreach ( $tags as $index => $tag_name ) {
+		if ( ! has_filter( "wp_filter_{$tag_name}_tags" ) || false === strpos( $content, "<$tag_name" ) ) {
+			unset( $tags[ $index ] );
+		}
+	}
 
 	if ( empty( $tags ) ) {
 		return $content;
 	}
 
+	// Expects well-formed HTML 5.0.
+	// Intended for `img` and `iframe`.
 	return preg_replace_callback(
-		'/<(' . implode( '|', $tags ) . ')(\s)[^>]+>/',
-		function( array $matches ) {
-			if ( ! preg_match( '/\sloading\s*=/', $matches[0] ) ) {
-				$tag   = $matches[1];
-				$space = $matches[2];
+		'/<(' . implode( '|', $tags ) . ')(?:\s[^>]+)?>/',
+		function( array $matches ) use ( $content, $context ) {
+			$tag_html = $matches[0];
+			$tag_name = $matches[1];
 
-				return str_replace( '<' . $tag . $space, '<' . $tag . $space . 'loading="lazy" ', $matches[0] );
-			}
-
-			return $matches[0];
+			/**
+			 * Filters each of the HTML tags that were found in $content.
+			 *
+			 * The variable part of the filter is the name of the matched tag.
+			 *
+			 * @since (TBD)
+			 *
+			 * @param string $tag_html The matched HTML markup.
+			 * @param string $content The (HTML) content being filtered.
+			 * @param string $context Optional. Additional context. May be the current filter name or specific context passed to the function.
+			 */
+			return apply_filters( "wp_filter_{$tag_name}_tags", $tag_html, $content, $context );
 		},
 		$content
 	);
 }
-