导航页面下拉菜单

<script>
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
            var subnav = this.getElementsByTagName(‘ul‘)[0];
            if(subnav){
                var This = subnav;
                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 subn = this.getElementsByTagName(‘ul‘)[0];
            if(subn) {
                var tt = subn;
                clearInterval(tt.time);
                tt.time = setInterval(function() {
                    tt.style.height = tt.offsetHeight -16 +"px";
                    if(tt.offsetHeight <= 0) 
                        clearInterval(tt.time);
                },30)
            }                        
                                                         
        }                
    }
}
</script>