bootstrap4.css
.embed-responsive .embed-responsive-item,
.embed-responsive iframe,
.embed-responsive embed,
.embed-responsive object,
.embed-responsive video {
position: absolute;
top: 0;
bottom: 0;
left: 0;
width: 100%;
height: 100%;
border: 0;
}
■ディスプレー:Display:ユーティリティ:Embedユーティリティ 埋め込みクラス アスペクト比 16:9
<div class="embed-responsive embed-responsive-16by9">
<iframe class="embed-responsive-item" src="https://www.youtube.com/embed/zpOULjyy-n8?rel=0" allowfullscreen></iframe>
</div>