HTML&CSS----练习隐藏导航栏（初级）

<!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=utf-8" />
<title>隐藏导航练习</title>
<style type="text/css">
<style type="text/css">
*
{
   margin:0px;
   padding:0px;}
#aa
{
    height:40px;
    width:90px;
    top:80px;
    left:200px;
    background-color:#339;
    position:absolute;
    overflow:hidden;
    text-align:center;
    line-height:40px;
}
#bb
{
    height:120px;
    width:90px;
    top:40px;
    left:0px;
    background-color:#3FC;
    position:absolute;
}
table
{
    height:120px;
    width:90px;
    border:0px;
    text-align:center;
}
</style>
</head>

<body>
<div id="aa" onmouseover="over()"  onmouseout="out()">新闻动态
<div id="bb">
<table cellpadding="0" cellspacing="0">
<tr><td>场景</td></tr>
<tr><td>活动</td></tr>
<tr><td>杂谈</td></tr>
</table>
<script>
function over()
{
 var a = document.getElementById("aa");
 a.setAttribute("style","overflow:visible;background-color:yellow");}
function out()
{
var a = document.getElementById("aa");
 a.setAttribute("style","overflow:hidden");
}
</script>
</div>
</div>
</body>
</html>

鼠标未放在上面时：

鼠标放在上面时：