Answers for "A fullscreen, responsive video with three overlayed, well-positioned buttons."

css background video

<div class="fullscreen-bg">
    <video loop muted autoplay poster="img/videoframe.jpg" class="fullscreen-bg__video">
        <source src="video/big_buck_bunny.webm" type="video/webm">
        <source src="video/big_buck_bunny.mp4" type="video/mp4">
        <source src="video/big_buck_bunny.ogv" type="video/ogg">
    </video>
</div>

<!-- CSS -->
.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%;
}

Background Video.module.css'

<video
  src="#defer-loading"
  poster="nice-default.jpg
  autoplay
/>

Background Video.module.css'

video.fullscreen { 
  position: fixed;
  top: 50%;
  left: 50%;
  min-width: 100%;
  min-height: 100%;
  width: auto;
  height: auto;
  z-index: -100;
  transform: translate(-50%, -50%);
}