Add lazy loading to oEmbed iframes (in WordPress)

add-lazy-loading-to-oembeds.php

<?php
/**
 * Add lazy loading to oEmbed iframes.
 *
 * @wordpress-plugin
 * Plugin Name: Lazy Load oEmbeds.
 * Description:  Add lazy loading to oEmbed iframes.
 * Plugin URI:
 * Version:     1.0.0
 * Author:      Adam Silverstein, Google
 * License:     Apache License 2.0
 * License URI: https://www.apache.org/licenses/LICENSE-2.0
 */

// See https://core.trac.wordpress.org/ticket/58773
function add_lazy_loading_attr_to_oembed_iframes( $iframe ) {

	// Skip oEmbeds without iframes.
	if ( ! preg_match_all( '/<(iframe)\s[^>]+>/', $iframe, $matches, PREG_SET_ORDER ) ) {
		return $iframe;
	}

	// Skip when lazy loading is disabled.
	if ( ! wp_lazy_loading_enabled( 'iframe', 'oembed' ) ) {
		return $iframe;
	}

	// Iterate through all iframes and add loading attribute (only one iframe is expected).
	foreach ( $matches as $match ) {
		$filtered_iframe = $match[0];

		// Add 'loading' attribute if applicable.
		if ( ! str_contains( $filtered_iframe, ' loading=' ) ) {
			$filtered_iframe = wp_iframe_tag_add_loading_attr( $filtered_iframe, 'oembed' );
		}

		if ( $filtered_iframe !== $match[0] ) {
			$iframe = str_replace( $match[0], $filtered_iframe, $iframe );
		}
		return $iframe;
	}
}
add_filter( 'embed_oembed_html', 'add_lazy_loading_attr_to_oembed_iframes' );