<div class="progress"></div>

<style>
  @keyframes go-left-right {        /* give it a name: "go-left-right" */
    from { left: 0px; }             /* animate from left: 0px */
    to { left: calc(100% - 50px); } /* animate to left: 100%-50px */
  }

  .progress {
    animation: go-left-right 3s infinite alternate;
    /* apply the animation "go-left-right" to the element
       duration 3 seconds
       number of times: infinite
       alternate direction every time
    */

    position: relative;
    border: 2px solid green;
    width: 50px;
    height: 20px;
    background: lime;
  }
</style>