div[data-background_youtubeid]{
    position:relative;
    overflow:hidden;
}

div[data-background_youtubeid] iframe, div[data-background_youtubeid] video{
    position: absolute;
    top: 50%;
    left: 50%;
    width: 100vw;
    height: 100vh;
    transform: translate(-50%, -50%);
    z-index: -1;
}

@media (min-aspect-ratio: 16/9) {
  div[data-background_youtubeid] iframe, div[data-background_youtubeid] video {
    /* height = 100 * (9 / 16) = 56.25 */
    height: 56.25vw;
  }
}
@media (max-aspect-ratio: 16/9) {
  div[data-background_youtubeid] iframe, div[data-background_youtubeid] video {
    /* width = 100 / (9 / 16) = 177.777777 */
    /*width: 177.78vh;*/
  }
}