html, body {
  margin: 0;
  height: 100%;
  background: #000;
}
body {
  /* clicking anywhere advances */
  cursor: pointer;
  -webkit-tap-highlight-color: transparent;
}
#bg {
  position: fixed;
  inset: 0;
  width: 100%;
  height: 100%;
  object-fit: cover;       /* fill screen, keep aspect */
  object-position: center; /* center crop */
  opacity: 1;
  transition: opacity .5s ease;
}
#bg.fading {
  opacity: 0;
}