在线演示;http://jsfiddle.net/Web_Code/ThhbG/embedded/result/
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<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="content-type" content="texthtml;charset=utf-8">
<title>Menu</title>
<style type="text/css">
#nav
{
list-style: none;
text-align: center;
}
#nav li
{
float: left;
100px;
color: white;
background-color: #3E3E3E;
}
#menu
{
list-style: none;
padding: 5px;
display: none;
margin-left: -5px;
margin-top: -5px;
}
#menu li
{
background-color: #ccc;
100px;
text-align: left;
padding-left: 10px;
}
#menu li a:link
{
text-decoration: none;
display: block;
}
#menu li a:hover
{
background-color:#3E3E3E;
color: white
}
</style>
</head>
<body>
<ul id="nav">
<li class="child">数 据 库
<ul id="menu">
<li><a href="http://blog.csdn.net/u011043843">MySQL</a></li>
<li><a href="http://blog.csdn.net/u011043843">SQL Server</a></li>
<li><a href="http://blog.csdn.net/u011043843">Oracle</a></li>
<li><a href="http://blog.csdn.net/u011043843">DB2</a></li>
</ul>
</li>
<li class="child">前台脚本
<ul id="menu">
<li><a href="http://blog.csdn.net/u011043843">JavaScript</a></li>
<li><a href="http://blog.csdn.net/u011043843">Ruby</a></li>
<li><a href="http://blog.csdn.net/u011043843">HTML</a></li>
<li><a href="http://blog.csdn.net/u011043843">Python</a></li>
</ul>
</li>
<li class="child">后台脚本
<ul id="menu">
<li><a href="http://blog.csdn.net/u011043843">PHP</a></li>
<li><a href="http://blog.csdn.net/u011043843">ASP</a></li>
<li><a href="http://blog.csdn.net/u011043843">ASP.NET</a></li>
<li><a href="http://blog.csdn.net/u011043843">JSP</a></li>
</ul>
</li>
</ul>
<script type="text/javascript">
var lis = document.getElementById("nav").getElementsByTagName('li');
var i = 0;
for( i = 0; i < lis.length; i++)
{
if(lis[i].className == "child")
{
lis[i].onmouseover = function()
{
var ulObj1 = this.getElementsByTagName('ul')[0];
ulObj1.style.display = "block";
this.style.backgroundColor="#ccc";
this.style.color="black";
}
}
lis[i].onmouseout = function()
{
var ulObj1 = this.getElementsByTagName('ul')[0]; //this是HTMLElement对象
ulObj1.style.display = "none";
this.style.backgroundColor="#3E3E3E";
this.style.color="white";
}
}
</script>
</body>
</html></span>