js 实现简单的导航下拉列表

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
<style>
    body,ul{margin:0;padding:0;}
    li{width:100px;height:30px;line-height:30px;float:left;border:1px solid #000;text-align:center;background:#00FFFF;list-style:none;position:relative;}
    li div{position:absolute;top:30px;left:0;filter:alpha(opacity=0);opacity:0;width:100%;    height:0;overflow:hidden;}
    /*这里的height:必须要写，overflow:也必须要写，不然没有下拉的效果*/
</style>
<script src="perfectStartMove.js"></script>
<script>
    window.onload = function(){
        var arrLi = document.getElementsByTagName('li');
        for(var i=0;i<arrLi.length;i++){
            arrLi[i].onmouseover = function(){
                var oDiv =     this.getElementsByTagName('div')[0];
                oDiv.style.height = 'auto';
                var heightVlaue = oDiv.offsetHeight;
                oDiv.style.height = 0;
                perfectStartMove(oDiv,{height:heightVlaue,opacity:100});
                
            };
            
            arrLi[i].onmouseout = function(){
                var oDiv =     this.getElementsByTagName('div')[0];
                perfectStartMove(oDiv,{height:0,opacity:0});
                
            };
        }
    };
</script>
</head>

<body>
    <ul>
        <li>
            首页
            <div>
                第三十<br />
                esfsdfds <br /> 
                dfdfd <br /> 
                fd
            </div>
        </li>
        <li>
            产品
            <div>
                fcd<br />
                esfsdfds <br /> 
                dfdfd
            </div>
        </li>
        <li>
            联系我们
            <div>
                读书三<br />
                ttt <br /> 
                dfdfd <br /> 
                fd
            </div>
        </li>
    </ul>
</body>
</html>