.video {
	position: absolute;
	top: 50%; 
	left: 50%;
	z-index: 1;
	min-width: 100%;
	min-height: 100%;
	width: auto;
	height: auto;
	transform: translate(-50%, -50%);
}

EDIT: Another method I found and am trying is this CSS based responsive video that keeps the 16/9 ratio.