<i></i>

i{
  position:fixed;
  width:50px;
  height:50px;
  border-radius:50%;
  background:radial-gradient(at 30% 30%,#0000,#000a) red;
  left:20px;
  top:20px;
  animation: x 0.3s cubic-bezier(0,4,1,4) infinite;
}

@keyframes x {
   to {
     top:20.1px;
   }
}