js+css制作下拉菜单没有效果出来
<!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>无标题文档</title>
<script language="javascript">
function showBar_left(li){
var ID=li.getElementByTagName("ul")[0];
ID.style.display="block";
}
function hidBar_left(li){
var ID=li.getElementByTagName("ul")[0];
ID.style.display="none";
}
</script>
<style>
#Bar_left li ul{ display:none}
</style>
</head>
<body>
<ul id="Bar_left">
<li onmouseover="showBar_left(this)" onmouseout="hidBar_left(this)">一级菜单1
<ul>
<li>2级菜单</li>
<li>2级菜单</li>
<li>2级菜单</li>
</ul>
</li>
<li onmouseover="showBar_left(this)" onmouseout="hidBar_left(this)">一级菜单2
<ul>
<li>2级菜单</li>
<li>2级菜单</li>
</ul>
</li>
</ul>
</body>
</html>
在线等,急!!