<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; right: 0; bottom: 0;
min- 100%; min-height: 100%;
auto; height: auto; z-index: -100;
background: url(polina.jpg) no-repeat;
background-size: cover;
---------------
<!--[if lt IE 9]>
<script>
document.createElement('video');
</script>
<![endif]-->
-----------
video { display: block; }
==============
http://www.webhek.com/video-background/