This post tests YouTube video embeds.
Simply wrap embeds with a <div> element and the appropriate classes:
<!-- 16:9 aspect ratio -->
<div class="embed-responsive embed-responsive-16by9">
<iframe class="embed-responsive-item" src="..."></iframe>
</div>
<!-- 4:3 aspect ratio -->
<div class="embed-responsive embed-responsive-4by3">
<iframe class="embed-responsive-item" src="..."></iframe>
</div>
<!-- responsive iframe. The framesize reduces proportionately when viewing in mobile -->
<div class="video-container">
<iframe class="embed-responsive-item" src="..."></iframe>
</div>