Видео на всю ширину и высоту экрана

~function () { 
  var video = document.querySelector('video') 
 
  function handleResize() { 
    var w0 = 320, h0 = 240, r0 = w0 / h0 
    var w = window.innerWidth, h = window.innerHeight, r = w / h 
     
    video.style.transform = r0 < r ? 'scaleX(' + r / r0 + ')' : 'scaleY(' + r0 / r + ')' 
     
  } 
 
  handleResize() 
  window.addEventListener('resize', handleResize) 
}()

video { 
  position: fixed; 
  top: 0; 
  left: 0; 
  right: 0; 
  bottom: 0; 
  width: 100%; 
  height: 100%; 
  z-index: -1; 
  transform-origin: center center; 
}

<video autoplay loop> 
  <source src="//www.w3schools.com/html/movie.mp4" type="video/mp4"> 
  <source src="//www.w3schools.com/html/movie.ogg" type="video/ogg"> 
  Your browser does not support the video tag. 
</video> 
 
<h1>Just some text</h1>