js版本下拉菜单

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<html>
<head>
<meta charset="utf-8">
<title>js版本下拉菜单</title>
<style>
*{ margin:0; padding:0; list-style:none;}
a{ text-decoration:none; color:#000;}
#top{ margin:0 auto; width:980px;}
.top-nav{ height:40px;}
.top-nav li{ float:left;width:200px; line-height:40px; background:#E5383B;}
.top-nav li a{ display:block; text-align:center;  color:#fff;}
.top-nav li a:hover{ background:#FC6D70; color:#F0F0F0;}
.top-nav li .se-nav{ display:none;}
.top-nav li .se-nav li{ height:45px; line-height:45px;}
</style>
<script type="text/javascript">
window.onload=function(){
	var oTop=document.getElementById('top');
	var oTopNav=document.getElementById('topNav');
	var aLi=document.getElementsByTagName('li');
	for(var i=0;i<aLi.length;i++){
		aLi[i].onmouseover=function(){
			var seNav=this.getElementsByTagName('ul')[0]
			if(seNav!=undefined){
				seNav.style.display='block';
				}
			}
		aLi[i].onmouseout=function(){
			var seNav=this.getElementsByTagName('ul')[0]
			if(seNav!=undefined){
				seNav.style.display='none';
				}
			}
		}
}
</script>
</head>
<body>
<div id="top">
    <ul class="top-nav" id="topNav">
        <li><a href="#">首页</a></li>
        <li>
        	<a href="#">课程大厅</a>
            <ul class="se-nav">
            	<li><a href="#">javascript</a></li>
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            </ul>
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        <li><a href="#">学习中心</a></li>
        <li>
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            <ul class="se-nav">
            	<li><a href="#">javascript</a></li>
                <li><a href="#">css</a></li>
                <li><a href="#">jquery</a></li>
            </ul>
        </li>
    </ul>

</div>
</body>
</html>