<!DOCTYPE html>
<html >
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
<title>erjidaohang</title>
<link rel="stylesheet" href="erjidaohang.css">
<script>
window.onload=function () {
var oUl=document.getElementById(‘ul1‘);
var aLi=oUl.children;/*请注意子节点只算第一层的，孙子节点不在子节点的范畴内。*/
for (var i=0;i<aLi.length;i++){
aLi[i].onmouseover=function () {
var oSubUl=this.getElementsByTagName(‘ul‘)[0];
if (oSubUl){
oSubUl.style.display="block";
}
};
/*在鼠标指针移出指定的对象时执行Javascript代码:*/
aLi[i].onmouseout=function () {
var oSubul=this.getElementsByTagName(‘ul‘)[0];
if (oSubul){
oSubul.style.display="none";
}
};
}
};
</script>
</head>
<body>
<div class="box">
<ul class="nav" id="ul1">
<li><a href="#">page1</a></li>
<li>
<a href="#">page2</a>
<ul>
<li><a href="#">item2</a></li>
<li><a href="#">item2</a></li>
<li><a href="#">item2</a></li>
<li><a href="#">item2</a></li>
</ul>
</li>
<li><a href="#">page3</a>
<ul>
<li><a href="#">item3</a></li>
<li><a href="#">item3</a></li>
<li><a href="#">item3</a></li>
<li><a href="#">item3</a></li>
</ul>
</li>
<li><a href="#">page4</a></li>
</ul>
</div>
</body>
CSS:
*{
margin: 0px;
padding: 0px;
list-style: none;
text-decoration: none;
}
.box{
width: 405px;
margin: 50px auto;
}
.box .nav{
margin-left: -1px;
}
.nav:after{
display: table;
content: "";
clear: both; /*清除左右浮动的影响*/
}
.nav>li{
float: left;
width: 100px;
margin-left: 1px; /*排除边界的影响*/
}
/*ransition-property是用来指定当元素其中一个属性改变时执行transition效果
，其中none没有属性改变；all所有属性都改变，它也是默认值；
indent元素属性名，其对应的类型如下：*/
.nav a{
display: block;
width: 100%;
height: 100%;
text-align: center;
border: 1px solid #000000;
color: #000000;
transition: .1s all ease;
}
.nav a:hover{
background-color: #eee;
color: #fff;
}
.nav li{
height: 30px ;
line-height: 30px;
}
.nav ul{
display:none ;
margin-top: -2px;
}
.nav ul li{
margin-top: 1px;
}
.active ul{
display: block;
}
原文：https://www.cnblogs.com/Damocless/p/11770265.html