window.onload=function(){
    var aLi=document.getElementsByTagName('li');
    for(var i=0; i<aLi.length; i++){
        aLi[i].onmouseover=function(){
            //鼠标经过一级菜单，二级菜单动画下拉显示出来
     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(){
                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){
                    This.style.height='0px';
                    clearInterval(This.time);
                }
             },30);
            }  
        }
    }
}