Generated by SassMeister.com.

SASS aspect ratio.css

section {
  width: 35%;
}
section div {
  /* Aspect ratio: 16:9 */
  height: 0;
  width: 100%;
  position: relative;
  padding-bottom: 56.25%;
}
section div > :first-child {
  position: absolute;
  z-index: 1;
  width: 100%;
  height: 100%;
}

SASS aspect ratio.html

<section>
  <div>
    <video controls>
      <source src="http://www.808.dk/vstreamer.asp?video=gizmo.mp4" />
    </video>
  </div>
  
  <!-- see the demo at http://sassmeister.com/gist/c868b13badace6638e29  -->
</section>

SASS aspect ratio.scss

// ----
// Sass (v3.4.14)
// Compass (v1.0.3)
// ----

@mixin ratio($arw, $arh) {
  /* Aspect ratio: #{$arw}:#{$arh} */
  height: 0; 
  width : 100%; 
  position: relative;
  padding-bottom: unquote("#{(round((100*$arh/$arw)*100))/100}%");
    
  > :first-child {
    position: absolute;
    z-index: 1;
    width: 100%;
    height: 100%;
  }
}


section {
  width: 35%;
  
  div {
    @include ratio(16,9);
  }
}