.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%;
}

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

This responsive video CSS trick was discovered by tjkdesign.com. I've blogged about it before, you may read the details here. It makes the video embed to expand fullwidth to the boundary.

Responsive Video

video

CSS