WordpressでYoutubeをレスポンシブ表示する

いろいろ試したけど以下が最適だった。
mediarealm.com.au

functions.phpに以下を追記

<?php
...（省略）

add_filter('the_content', function($content) {
	return str_replace(array("<iframe", "</iframe>"), array('<div class="iframe-container"><iframe', "</iframe></div>"), $content);
});

add_filter('embed_oembed_html', function ($html, $url, $attr, $post_id) {
	if(strpos($html, 'youtube.com') !== false || strpos($html, 'youtu.be') !== false){
  		return '<div class="embed-responsive embed-responsive-16by9">' . $html . '</div>';
	} else {
	 return $html;
	}
}, 10, 4);


add_filter('embed_oembed_html', function($code) {
  return str_replace('<iframe', '<iframe class="embed-responsive-item" ', $code);
});

CSSに以下を追加

.embed-responsive {
  position: relative;
  display: block;
  height: 0;
  padding: 0;
  overflow: hidden;
}
.embed-responsive .embed-responsive-item,
.embed-responsive iframe {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  width: 100%;
  height: 100%;
  border: 0;
}
.embed-responsive-16by9 {
  padding-bottom: 56.25%;
}