W3School TIY Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
body {margin: 0;}

ul.sidenav {
  list-style-type: none;
  margin: 0;
  padding: 0;
  width: 25%;
  background-color: #f1f1f1;
  position: fixed;
  height: 100%;
  overflow: auto;
}

ul.sidenav li a {
  display: block;
  color: #000;
  padding: 8px 16px;
  text-decoration: none;
}
 
ul.sidenav li a.active {
  background-color: #4CAF50;
  color: white;
}

ul.sidenav li a:hover:not(.active) {
  background-color: #555;
  color: white;
}

div.content {
  margin-left: 25%;
  padding: 1px 16px;
  height: 1000px;
}

@media screen and (max-width: 900px) {
  ul.sidenav {
    width: 100%;
    height: auto;
    position: relative;
  }
  
  ul.sidenav li a {
    float: left;
    padding: 15px;
  }
  
  div.content {margin-left: 0;}
}

@media screen and (max-width: 400px) {
  ul.sidenav li a {
    text-align: center;
    float: none;
  }
}
</style>
</head>
<body>

<ul class="sidenav">
  <li><a class="active" href="#home">Home</a></li>
  <li><a href="#news">News</a></li>
  <li><a href="#contact">Contact</a></li>
  <li><a href="#about">About</a></li>
</ul>

<div class="content">
  <h1>响应式侧导航栏实例</h1>
  <p>当屏幕尺寸为 900 像素或更小时，此例使用媒体查询将 sidenav 转换为顶部导航栏。</p>
  <p>我们还为屏幕小于等于 400 像素的屏幕添加了媒体查询，它将垂直堆叠并居中放置导航链接。</p>
  <p>您稍后将在我们的 CSS 教程中学到有关媒体查询和响应式 Web 设计的更多知识。</p>
  <p><b>请调整浏览器窗口的大小以查看效果。</b></p>
</div>

</body>
</html>