<meta charset="UTF-8">
<title>Document</title>
<style>
*{margin:0;padding: 0;list-style: none;text-decoration: none;}
#nav{width: 500px;height: 40px;background: #ccc;margin: 0 auto;}
ul{background: #aaa}
ul li{float:left; display:block; height: 40px; line-height: 40px; padding: 0 20px; position: relative;}
ul li:hover{background: #cea;}
ul li ul li{float: none;}
/*关键一：将二级菜单设置为display：none;*/
ul li ul{position: absolute;top:40px;left: 0; display: none;}
ul li ul li:hover{background: red;}
/*关键二：在划过二级菜单从属的一级菜单时，设置为display:block;*/
ul li:hover ul{display: block;}
</style>
 
 
<div id="nav">
  <ul>
    <li><a href="">首页</a></li>
    <li><a href="">汽车</a>
      <ul>
        <li><a href="#">奥迪</a> </li>
        <li><a href="#">道奇</a> </li>
      </ul>
    </li>
    <li><a href="">手机</a>
      <ul>
        <li><a href="#">小米</a> </li>
        <li><a href="#">华为</a> </li>
      </ul>
    </li>
    <li><a href="">联系我们</a></li>
  </ul>
</div> 

<meta charset="UTF-8">
<title>Document</title>
<style>
*{margin:0;padding: 0;list-style: none;text-decoration: none;}
#nav{width: 500px;height: 40px;background: #ccc;margin: 0 auto;}
ul{background: #aaa}
ul li{float:left; display:block; height: 40px; line-height: 40px; padding: 0 20px; position: relative;}
ul li:hover{background: #cea;}
ul li ul li{float: none;}
ul li ul{position: absolute;top:40px;left: 0; display:none;}
ul li ul li:hover{background: red;}
</style>
 
 
<div id="nav">
  <ul>
    <li><a href="#">首页</a></li>
    <li onmouseover="show(this)" onmouseout="hide(this)"><a href="#">汽车</a>
  <!-- 关键一：在二级标题从属的一级标题标签内设置时间执行程序，this代表的时这个li元素 -->
      <ul>
        <li><a href="#">奥迪</a> </li>
        <li><a href="#">道奇</a> </li>
      </ul>
    </li>
    <li onmouseover="show(this)" onmouseout="hide(this)"><a href="#">手机</a>
      <ul>
        <li><a href="#">小米</a> </li>
        <li><a href="#">华为</a> </li>
      </ul>
    </li>
    <li><a href="#">联系我们</a></li>
  </ul>
</div> 

<meta charset="UTF-8">
<title>Document</title>
<style>
*{margin:0;padding: 0;list-style: none;text-decoration: none;}
#nav{width: 500px;height: 40px;background: #ccc;margin: 0 auto;}
ul{background: #aaa}
ul li{float:left; display:block; height: 40px; line-height: 40px; padding: 0 20px; position: relative;}
ul li:hover{background: #cea;}
ul li ul li{float: none;}
ul li ul{position: absolute;top:40px;left: 0; display: none;}
ul li ul li:hover{background: red;}
</style>
 
 
<div id="nav">
  <ul>
    <li><a href="">首页</a></li>
    <li class="navmenu"><a href="">汽车</a>
      <ul>
        <li><a href="#">奥迪</a> </li>
        <li><a href="#">道奇</a> </li>
      </ul>
    </li>
    <li class="navmenu"><a href="">手机</a>
      <ul>
        <li><a href="#">小米</a> </li>
        <li><a href="#">华为</a> </li>
      </ul>
    </li>
    <li><a href="">联系我们</a></li>
  </ul>
</div>