<div class="container">
  <div class="auto-scale">
    <iframe
            src="//www.flickr.com/photos/rillke/14939106903/in/set-72157646495502983/player/"
            frameborder="0"
            allowfullscreen webkitallowfullscreen mozallowfullscreen oallowfullscreen msallowfullscreen>
    </iframe>
  </div>
  <div style="padding-top: 42.3%;" class="ratio-holder"></div>
</div>

.container {
    width: 100%; /* Set this to the width you prefer */
    display: inline-block;
    position: relative;
    background-color: #eee;
}
.auto-scale {
    position: absolute;
    top: 0;
    bottom: 0;
    right: 0;
    left: 0;
}
.auto-scale>iframe {
    width: 100%;
    height: 100%;
}