老师在哪里，我遇到了问题求解答
<!DOCTYPE HTML>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
.top-nav
{
font-size: 14px;
font-weight: bold;
list-style-type: none;
}
.top-nav li
{
float: left;
margin-right: 1px;
}
.top-nav li a
{
line-height: 34px;
text-decoration: none;
background: #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/button.png)0 -2px repeat-x;
}
.note
{
color: #3f240e;
display: block;
background:url(./img/button.png)0 -2px repeat-x;
}
.corner
{
display: block;
height: 11px;
background:url(./img/corner.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){
var uList=document.getElementById(id);
var h=uList.offsetHeight;
h+=1;
if(h<=45){
uList.style.height=h+"px";
setTimeout("AddH('"+id+"')",10);
}
else{
return;
}
}
function SubH(id){
var uList=document.getElementById(id);
var h=uList.offsetHeight;
h-=1;
if(h>0){
uList.style.height=h+"px";
setTimeout("SubH('"+id+"')",10);
}
else{
ulList.style.display="none";
return;
}
}
</script>
</head>
<body>
<ul class="top-nav">
<li>
<a href="#">
<span>慕课网</span>
</a>
</li>
<li>
<a href="#">课程大厅</a>
</li>
<li>
<a href="#">学习中心</a>
<ul id="menuL">
<span class="corner"></span>
<li>
<a href="#">前端课程</a>
</li>
<li>
<a href="#">手机开发</a>
</li>
<li>
<a href="#">后台开发</a>
</li>
</ul>
</li>
<li>
<a href="#">关于我们</a>
</li>
</ul>
</body>
</html>
