Edit in JSFiddle

<div class="youtube">
    <iframe src="http://www.youtube.com/embed/jICRkfcOOD0" frameborder="0" width="100%" height="100%"></iframe>
</div>

.youtube {
    position: relative;
    padding-bottom: 56.25%;
    padding-top: 30px; height: 0; overflow: hidden;
}

.youtube iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}