jaaniic'ša pierakstu klade

<video autoplay loop poster="polina.jpg" id="bgvid">
<source src="polina.webm" type="video/webm">
<source src="polina.mp4" type="video/mp4">
</video>

video#bgvid {
position: fixed;
top: 50%;
left: 50%;
min-width: 100%;
min-height: 100%;
width: auto;
height: auto;
z-index: -100;
-webkit-transform: translateX(-50%) translateY(-50%);
transform: translateX(-50%) translateY(-50%);
background: url(polina.jpg) no-repeat;
background-size: cover;
}

@media screen and (max-device-width: 800px) {
body { background: url(polina.jpg) #000 no-repeat center center fixed; }
#bgvid { display: none; }
}

http://demosthenes.info/blog/777/Create-Fullscreen-HTML5-Page-Background-Video