Responsive Youtube Embed Video

<!-- 21:9 aspect ratio -->
<div class="embed-responsive embed-responsive-21by9">
  <iframe class="embed-responsive-item" src="..."></iframe>
</div>

<!-- 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>

<!-- 1:1 aspect ratio -->
<div class="embed-responsive embed-responsive-1by1">
  <iframe class="embed-responsive-item" src="..."></iframe>
</div>