div#wrapper {
  background-image: url('i/flake1.png'), url('i/flake2.png'), url('i/flake3.png');
  background-repeat: no-repeat;
  height: 100vh;
  animation: snow 20s linear infinite;
}

@keyframes snow {
    0% {background-position: 10% 1rem, 50% 5rem, 80% 2rem;}
    100% {background-position: 80% 100%, 10% 100%, 50% 100%;}
}