.video {
position: relative;
padding-bottom: 56.25%;
height: 0;
overflow: hidden;
}
.video iframe,
.video object,
.video embed {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
}
<p class="video"><iframe src="http://player.youku.com/embed/XOTI3MjU1MTIw" frameborder="0" allowfullscreen=""></iframe></p>