Bootstrap class: .embed-responsive

<div class="embed-responsive embed-responsive-16by9">
  <iframe class="embed-responsive-item" src="//www.youtube.com/embed/48OJbbI0DfE" allowfullscreen></iframe>
</div>

Preview

Sass source

/* _embed.scss:3 */
.embed-responsive {
  position: relative;
  display: block;
  width: 100%;
  padding: 0;
  overflow: hidden;

  &::before {
    display: block;
    content: "";
  }

  .embed-responsive-item,
  iframe,
  embed,
  object,
  video {
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    width: 100%;
    height: 100%;
    border: 0;
  }
}

Uses variables

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/* _embed.scss:3 */

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