:root::before {
  content:"";
  position: absolute;
  top: 50%;
  left: 50%;
  translate: -50% -50%;
  width: 70vmin;
  aspect-ratio: 2;
  background:
    linear-gradient(90deg, #fff0 25%, #f8f8f8 50%, #666 0, #0000 75%) 0 0 / 100% 100%,
    #bbb;
  background-repeat: no-repeat;
  transition: background-position 1s;
}

:root:hover::before {
  background-position: 0 30vmin;
}