#button .fa-arrow-down {
  position: absolute;
  top: 16rem;
  left: 15rem;
  color: orangered;
  font-size: 4rem;
  animation: start 1.5s none ease-in-out infinite;
}
@keyframes start {
  25% {
    top: 30%;
  }
  50% {
    top: 40%;
  }
  75% {
    top: 25%;
  }
  100% {
    top: 37%;
  }
}