text: <!DOCTYPE html>
  <html lang="en">
  <head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
      /* https://developer.mozilla.org/zh-CN/docs/Web/CSS/animation */
      .container {
        width: 100px;
        height: 100px;
        background: red;
        animation: run 1s;
        animation-direction: reverse;
        animation-fill-mode: forwards;
        animation-iteration-count: infinite;
        animation-play-state: paused;
      }
      @keyframes run {
        0% {
          width: 100px;
        }
        50% {
          width: 800px;
        }
        100% {
          width: 100px;
        }
      }
    </style>
  </head>
  <body>
    <div class="container"></div>
  </body>
  </html>
  