HTML5-video-stretch-fullscreen.css

// http://fvsch.com/code/video-background/

#video-bg {
  position: fixed;
  top: 0; right: 0; bottom: 0; left: 0;
  overflow: hidden;
}
#video-bg > video {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}
/* 1. No object-fit support: */
@media (min-aspect-ratio: 16/9) {
  #video-bg > video { height: 300%; top: -100%; }
}
@media (max-aspect-ratio: 16/9) {
  #video-bg > video { width: 300%; left: -100%; }
}
/* 2. If supporting object-fit, overriding (1): */
@supports (object-fit: cover) {
  #video-bg > video {
    top: 0; left: 0;
    width: 100%; height: 100%;
    object-fit: cover;
  }
}