.left {
  animation-duration: 3s;
  animation-name: first;
  width: 25%;
  float: left;
  height: 100%;
  background: pink;
}

@keyframes first {
  from {
    margin-left: 100%;
    width: 300%; 
  }

  to {
    margin-left: 0%;
    width: 100%;
  }
}