Le Merveilleux Blog
Responsive Design : Video & other container fluid

.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%;
}

source : http://webdesignerwall.com/tutorials/5-useful-css-tricks-for-responsive-design