这是 CSS 代码 - 在线编辑器

          <!DOCTYPE html>
<html>
<head>
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta charset="utf-8">
  <style>
    div {
      width: 100%;
      height: 400px;
      background-image: url('/wp-content/uploads/2015/06/img_flowers.jpg');
      background-repeat: no-repeat;
      background-size: contain;
      border: 1px solid red;
    }
  </style>
</head>
<body>

  <p>调整浏览器大小查看效果。</p>

  <div></div>

</body>
</html>
        