#stars, #stars2, #stars3 {
  position: absolute;
  background: transparent;
  z-index: -1;
  animation: animStar linear infinite;
}

#stars {
  width: 1px;
  height: 1px;
  animation-duration: 50s;
}

#stars2 {
  width: 2px;
  height: 2px;
  animation-duration: 100s;
}

#stars3 {
  width: 3px;
  height: 3px;
  animation-duration: 150s;
}

@keyframes animStar {
  from {
    transform: translateY(0);
  }
  to {
    transform: translateY(-2000px);
  }
}