A Pen by Adam Onishi.

index.html

<div class="video-wrapper">
  <iframe src="//www.youtube.com/embed/bVKCHZqax84" frameborder="0" allowfullscreen></iframe>
</div>

lybwA.markdown

lybwA

A Pen by Adam Onishi on CodePen.

License.

style.css

.video-wrapper {
  position:relative;
  width:100%;
  height:0;
  padding-top:25px;
  padding-bottom:100%; /* default */
}

@media screen and (min-aspect-ratio: 4/3) {
  .video-wrapper{
    padding-bottom:75%;
  }
}

@media screen and (min-aspect-ratio: 16/9) {
  .video-wrapper{
    padding-bottom:56.25%; /* 16:9 */    
  }
}

.video-wrapper iframe {
    position:absolute;
    top:0;
    left:0;
    width:100%;
    height:100%;
}