Set aspect ratio for the iframe with WP_HTML_Tag_Processor

iframe-aspect-ratio.php

<?php
/**
 * Set aspect ratio for the iframe.
 */
$tags = new WP_HTML_Tag_Processor( $html );

if ( $tags->next_tag( 'iframe' ) ) {
	$width  = $tags->get_attribute( 'width' );
	$height = $tags->get_attribute( 'height' );
	$style  = $tags->get_attribute( 'style' );

	$tags->set_attribute( 'width', '100%' );
	$tags->remove_attribute( 'height' );
	$tags->set_attribute( 'style', $style . "; aspect-ratio: {$width}/{$height};" );
}

echo $tags->get_updated_html();