Fluid Video Embeds. Includes modest branding and other niceties in the embed URL.

content.html

<div class="fve-video-wrapper">
  <iframe width="100%" height="100%" src="https://www.youtube.com/embed/kCIRQuHdBgQ?wmode=transparent&amp;modestbranding=1&amp;autohide=1&amp;showinfo=0&amp;rel=0" frameborder="0" allowfullscreen></iframe>
</div>

styles.scss

  .fve-video-wrapper {
    position: relative;
    overflow: hidden;
    height: 0;
    padding-bottom: 56.25%;
    margin: 1.5em 0; /* A bit of margin at the bottom */

    iframe,
    object,
    embed {
      position: absolute;
      display: block;
      top: 0;
      left: 0;
      width: 100%;
      height: 100%;
    }
  }