先执行一遍动画，然后再反向执行一遍动画 - 在线编辑器

          <!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <style>
    div {
      width: 100px;
      height: 100px;
      background: red;
      position: relative;
      animation: myfirst 5s infinite;
      animation-direction: alternate;

      /* Safari 和 Chrome */
      -webkit-animation: myfirst 5s infinite;
      -webkit-animation-direction: alternate;
    }

    @keyframes myfirst {
      0% {
        background: red;
        left: 0px;
        top: 0px;
      }
      25% {
        background: yellow;
        left: 200px;
        top: 0px;
      }
      50% {
        background: blue;
        left: 200px;
        top: 200px;
      }
      75% {
        background: green;
        left: 0px;
        top: 200px;
      }
      100% {
        background: red;
        left: 0px;
        top: 0px;
      }
    }

    @-webkit-keyframes myfirst
    /* Safari 和 Chrome */

    {
      0% {
        background: red;
        left: 0px;
        top: 0px;
      }
      25% {
        background: yellow;
        left: 200px;
        top: 0px;
      }
      50% {
        background: blue;
        left: 200px;
        top: 200px;
      }
      75% {
        background: green;
        left: 0px;
        top: 200px;
      }
      100% {
        background: red;
        left: 0px;
        top: 0px;
      }
    }
  </style>
</head>
<body>

  <p><strong>注意：</strong>Internet Explorer 9 及其之前的版本不支持 animation-direction 属性。</p>

  <div></div>

</body>
</html>
        