Responsive iframe

<!-- 21:9 aspect ratio -->
<div class="ratio ratio-21x9">
  <iframe src="..."></iframe>
</div>

<!-- 16:9 aspect ratio -->
<div class="ratio ratio-16x9">
  <iframe src="..."></iframe>
</div>

<!-- 4:3 aspect ratio -->
<div class="ratio ratio-4x3">
  <iframe src="..."></iframe>
</div>

<!-- 1:1 aspect ratio -->
<div class="ratio ratio-1x1">
  <iframe src="..."></iframe>
</div>