1

HTML

<div class="fullframe">
  <iframe src="https://www.youtube.com/embed/BlMZfiGzzxA" frameborder="0" allowfullscreen></iframe>
</div>

CSS

.fullframe {
  height: 0;
  margin: 0;
  overflow: hidden;
  padding-bottom: 50%;
  position: relative;
  z-index: 0;
}
.fullframe > iframe, .fullframe > object, .fullframe > embed {
  border: 0;
  height: 100%;
  left: 0;
  position: absolute;
  top: 0;
  width: 100%;
  z-index: -1;
}
iframe {
  max-width: 100%;
}