div {
  width: 100px;
  height: 100px;
  border-radius: 50%;
  background-color: red;
  animation-name: example;
  animation-duration: 10s;
  animation-iteration-count: infinite;
  animation-timing-function: linear;
}

@keyframes example {
  0%   {background-color:red; left:0px; top:50%;}
  50%  {background-color:red; left:calc(100vw - 100px); top:50%;}
  100% {background-color:red; left:0px; top:50%;}
}