body {
  /* these don't matter: */
  margin: 0;
  padding: 0;
}
.full-window-height {
  /*height: 100vh; /* ONLY if you want the video to fill the entire browser window. Support: http://caniuse.com/#search=vh */
}
.video-bg {
  position: relative; /* contain the abs-pos <video> */
  width: 100%;
  overflow: hidden; /* prevent scrollbars */
  /* these below don't matter: */
  /*padding: 10% 0; */
  text-align: center;
  font: bold 40px/1.2 sans-serif;
  margin-bottom: -1rem;
}
.video-bg video {
  /*position: absolute;*/
  top: 50%;
  left: 50%;
  /*transform: translate(-50%, -50%); /* keep centered in parent */
  z-index: -1; /* keep video behind other content */
  min-width: 100%; /* always cover horizontal */
  min-height: 100%; /* always cover vertical */
}

.full-window-height.video-bg.tablet,
.full-window-height.video-bg.mobile{
    display: none;
  }

@media (max-width: 834px) {
  .full-window-height.video-bg.desktop {
    display: none;
  }
  .full-window-height.video-bg.tablet {
    display: block;
  }
}
@media (max-width: 400px) {
  .full-window-height.video-bg.mobile {
    display: block;
  }
    .full-window-height.video-bg.tablet {
    display: none;
  }
}