以下实例在屏幕可视窗口尺寸大于 480 像素时将菜单浮动到页面左侧 - 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>
    .wrapper {
      overflow: auto;
    }

    #main {
      margin-left: 4px;
    }

    #leftsidebar {
      float: none;
      width: auto;
    }

    #menulist {
      margin: 0;
      padding: 0;
    }

    .menuitem {
      background: #CDF0F6;
      border: 1px solid #d4d4d4;
      border-radius: 4px;
      list-style-type: none;
      margin: 4px;
      padding: 2px;
    }

    @media screen and (min-width: 480px) {
      #leftsidebar {
        width: 200px;
        float: left;
      }
      #main {
        margin-left: 216px;
      }
    }
  </style>
</head>
<body>

  <div class="wrapper">
    <div id="leftsidebar">
      <ul id="menulist">
        <li class="menuitem">Menu-item 1</li>
        <li class="menuitem">Menu-item 2</li>
        <li class="menuitem">Menu-item 3</li>
        <li class="menuitem">Menu-item 4</li>
        <li class="menuitem">Menu-item 5</li>
      </ul>
    </div>
    <div id="main">
      <h1>重置浏览器窗口查看效果！</h1>
      <p>在屏幕可视窗口尺寸大于 480 像素时将菜单浮动到页面左侧。</p>
    </div>
  </div>

</body>
</html>
        