@keyframes anim {
  100% { background-image: url("green-20x20.png"); }
}

@keyframes ref {
  100% { background-image: linear-gradient(to bottom, lime, lime); }
}

* {
  background: linear-gradient(to bottom, red, red);
  animation: anim steps(5,end) 20s;
}

#reference {
  animation-name: ref;
}