html二级菜单动画逐渐收缩或显示

动画效果类似这个网站的（= =不是广告，找不到其他的了）
http://www.17sucai.com/pins/demoshow/242
具体实现：
<!DOCTYPE html>  
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>动画菜单</title>
<style type="text/css">
* { margin: 0; padding: 0}
a { color: #333; text-decoration: none }
ul{ list-style: none; }
.nav {height: 30px; margin-left:50px; width:600px;}
.nav li { float: left; position:relative; height:30px; width:120px }
.nav li a { display: block; height: 30px; text-align: center; line-height: 30px; width:120px; background: #efefef; margin-left: 1px; }
.subNav{ position:absolute; top:30px; left:0; width:120px; height:0; overflow:hidden}
.subNav li a{ background:#ddd }
.subNav li a:hover{ background:#efefef}
</style>
<script>
window.onload=function(){
    var aLi=document.getElementsByTagName('li');
	for(var i=0; i<aLi.length; i++){
	aLi[i].onmouseover=function(){
            var oSubNav=this.getElementsByTagName('ul')[0];
            if(oSubNav){
            var This=oSubNav;
            clearInterval(This.time);
            This.time=setInterval(function(){
                    This.style.height=This.offsetHeight+16+"px";
                    if(This.offsetHeight>=120)
                    clearInterval(This.time);
                },30)
             }
          }
        //鼠标离开菜单，二级菜单动画收缩起来。 
 aLi[i].onmouseout=function(){
            var oSubNav=this.getElementsByTagName('ul')[0];
            if(oSubNav){
            var This=oSubNav;
            clearInterval(This.time);
            This.time=setInterval(function(){
                    This.style.height=This.offsetHeight-16+"px";
                    if(This.offsetHeight<=0)
                    clearInterval(This.time);
                },30)
             }
          }
	}
}
</script>
</head>
<body>
<ul class="nav">
    <li><a href="#">一级菜单</a>
    <ul class="subNav">
        <li><a href="#">二级菜单</a></li>
            <li><a href="#">二级菜单</a></li>
            <li><a href="#">二级菜单</a></li>
            <li><a href="#">二级菜单</a></li>
        </ul>
    </li>
    <li><a href="#">一级菜单</a>
    <ul class="subNav">
        <li><a href="#">二级菜单</a></li>
            <li><a href="#">二级菜单</a></li>
            <li><a href="#">二级菜单</a></li>
            <li><a href="#">二级菜单</a></li>
        </ul>
    </li>
</ul>
</body>
</html>
html二级菜单动画逐渐收缩或显示

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