<div class="div1"></div>
<div class="div2"></div>

.div1 {
  height: 20px;
  background-color: green;
  animation: widthAnim 10s infinite;
}

.div2{
  height: 20px;
  background-color: green;
  animation: widthAnim 10s steps(10, jump-none) infinite;
}



@keyframes widthAnim {
  0%, 100% {
    width: 0;
  }
  50% {
    width: 100%;
  }
}