JSFiddle - React, Tailwind, and code Playground

HTML

<div id="video-bg">
  <video controls="" loop="">
    <source type="video/mp4" src="http://grochtdreis.de/fuer-jsfiddle/video/sintel_trailer-480.mp4">
  </video>
</div>

CSS

#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;
    }
  }