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<span id="box"></span>

#box{
  display:block;
  width:10px;
  height:100px;
  background:gold;
  animation:2s ease-in 1s infinite alternate forwards running expansion;
}
@keyframes expansion{
  0%{
    width:10px;
  }
  100%{
    width:300px;
    background:tomato;
  }
}

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