<!-- Basically you just add your video here...before anything content -->

<div class="fullscreen-bg">
  <video loop muted autoplay class="fullscreen-bg__video">
      <source src="video.mp4" type="video/mp4">
  </video>
</div>

<!-- Then you add some CSS and voilá... -->

<style>
  .fullscreen-bg {
    position: fixed;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    overflow: hidden;
    z-index: -100;
  }

  .fullscreen-bg__video {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
  }

  @media (min-aspect-ratio: 16/9) {
    .fullscreen-bg__video {
      height: 300%;
      top: -100%;
    }
  }

  @media (max-aspect-ratio: 16/9) {
    .fullscreen-bg__video {
      width: 300%;
      left: -100%;
    }
  }

  @media (max-width: 767px) {
    .fullscreen-bg__video {
      display: none;
    }
  }
</style>

<!-- You have a nice video background image then -->