下拉菜单不出来，还有这种报错
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
body{
behavior:url(csshover.htc);
}
/**{margin: 0;padding: 0;}*/
.top-nav{font-size: 14px;font-weight: bold;list-style: none;}
.top-nav li{float: left;margin-right: 1px;}
.top-nav li a{line-height: 34px;text-decoration: none;background-color: #3f240e;color: #fff;display: block;width: 80px;text-align: center;}
.top-nav ul{list-style: none;display: none;padding: 0;position: absolute;height: 0;overflow: hidden;}
.top-nav li a:hover{background: url(img/slide.png) 0 0 repeat-x;}
.note{color: #3f240e;display: block;background: url(img/slide.png) 0 0 repeat-x;}
.corner{display: block;height: 11px;background: url(img/bird.png) 31px 0 no-repeat;}
</style>
<script type="text/javascript">
window.onload=function(){
var Lis = document.getElementsByTagName('li');
for (var i = 0; i < Lis.length; i++) {
Lis[i].onmouseover = function(){
var u = this.getElementsByTagName("ul")[0];
if(u=!undefined){
u.style.display='block';
AddH(u.id);
}
}
Lis[i].onmouseleave = function(){
var u = this.getElementsByTagName("ul")[0];
if(u=!undefined){
SubH(u.id);
}
}
};
}
function AddH (id) {
// body...
var ulList = document.getElementById(id);
var h= ulList.offsetHeight;
h+=1;
if (h<=42) {
ulList.style.height = h+"px";
setTimeout("AddH('"+id+"')",10);
} else{
return;
};
}
function SubH(){
}
</script>
</head>
<body>
<ul id="nav">
<li><a href=""><span>首页</span></a></li>
<li><a href="">学习中心</a>
<ul id="menuUL">
<span></span>
<li><a href="">前端课程</a></li>
<li><a href="">手机开发</a></li>
<li><a href="">后台编程</a></li>
</ul>
</li>
<li><a href="">经典案例</a></li>
<li><a href="">关于我们</a></li>
</ul>
</body>
</html>
下拉菜单不出来，还有这种报错
