Generated by SassMeister.com.

SassMeister-input-HTML.html

<h2>Video with fitVid class</h2>
<div class="fitVid">
  <iframe width="560" height="315" src="//www.youtube.com/embed/kK0_OVn_uxA" frameborder="0" allowfullscreen></iframe>
</div>
  
<h2>Video with without the class</h2>
<iframe width="560" height="315" src="//www.youtube.com/embed/kK0_OVn_uxA" frameborder="0" allowfullscreen></iframe>

SassMeister-input.sass

// ----
// Sass (v3.3.0.rc.2)
// Compass (v1.0.0.alpha.17)
// ----

.fitVid
  position: relative
  height: 0
  padding-top: 56.25% // 16-9 ratio
  // padding-top: 50% // 2-1 (vimeo)
  // padding-top: 100% // 1-1
  // padding-top: 75% // 4-3

  iframe
    position: absolute
    z-index: 100 // tweak this
    top: 0
    right: 0
    bottom: 0
    left: 0
    width: 100%
    height: 100%

    
    
// Generic stuff
body
  max-width: 560px
iframe
  max-width: 100%

SassMeister-output.css

.fitVid {
  position: relative;
  height: 0;
  padding-top: 56.25%;
}
.fitVid iframe {
  position: absolute;
  z-index: 100;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  width: 100%;
  height: 100%;
}

body {
  max-width: 560px;
}

iframe {
  max-width: 100%;
}

SassMeister-rendered.html

<h2>Video with fitVid class</h2>
<div class="fitVid">
  <iframe width="560" height="315" src="//www.youtube.com/embed/kK0_OVn_uxA" frameborder="0" allowfullscreen></iframe>
</div>
  
<h2>Video with without the class</h2>
<iframe width="560" height="315" src="//www.youtube.com/embed/kK0_OVn_uxA" frameborder="0" allowfullscreen></iframe>