.container{
  width: 100px;
  height: 100px;
  background: red;
  animation: run 1s linear;
  /* animation-direction: reverse; */
  /* animation-fill-mode: forwards; */
  animation-iteration-count: infinite;
  /* animation-play-state: paused; */
}
@keyframes run{
  0%{
    width: 100px;
  }
  50%{
    width: 800px;
  }
  100%{
    width: 100px;
  }
}