Full width and height video background CSS

background-video-responsive.css

.bg-video {
  position: fixed;
  z-index: -1;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
}

@media (min-aspect-ratio: 16/9) {
  .bg-video video {
    width: 100%;
    height: auto;
  }
}

@media (max-aspect-ratio: 16/9) {
  .bg-video video {
    width: auto;
    height: 100%;
  }
}

video.html

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