Html5 video background, keep center of video in center

here’s how I typically do background video, and how I did it for the stre.am landing page:

.video_contain {
    position: absolute;
    top: -50%;
    left: -50%;
    width: 200%;
    height: 200%;
}

video {
    position: absolute;
    top: 0;
    bottom: 0;
    right: 0;
    left: 0;
    margin: auto;
    min-height: 50%;
    min-width: 50%;
}