查看实际效果：http://keleyi.com/a/bjac/vksd7321.htm
完整代码，保存在html文件打开也可看到效果：
<!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>CSS导航条-柯乐义</title><base target="_blank" />
<style>
ul,li{ margin:0px; padding:0px; list-style:none;}
.div_a{ 300px; float:left;}
.div_a li{ 300px; height:50px; border-bottom:1px solid #fff; cursor:pointer; background-color:#060; text-align:center; line-height:50px; font-size:18px; color:#fff;}
.div_a1-keleyi-com{ 300px; height:100px; position:relative; background-color:#F00; top:-50px; left:250px; z-index:999; display:none;}
.div_a li:hover .div_a1-keleyi-com{ display:block;}.div_a a{ color:white;}
</style>
</head>
<body>
<div class="div_a">
<ul>
<li><a href="http://keleyi.com/menu/aspnet/">ASP.NET</a><div class="div_a1-keleyi-com"><a href="http://keleyi.com/a/bjac/kjsrt3b0.htm">基于微软.NET的编程平台</a></div></li>
<li><a href="http://keleyi.com/a/bjac/ck9atu5j.htm">PHP</a><div class="div_a1-keleyi-com"><a href="http://keleyi.com/a/bjac/mnmpm4bv.htm">使用最多的WEB开发语言</a></div></li>
<li><a href="http://keleyi.com/a/bjac/kjsrt3b0.htm">AJAX</a><div class="div_a1-keleyi-com"><a href="http://keleyi.com/a/bjac/182di68b.htm">实现网页交互式设计</a><br /><a href="http://keleyi.com/a/bjac/13639353acde40c.htm">左侧可弹出隐藏div</a></div></li>
<li><a href="http://keleyi.com/menu/csharp/">C#.NET</a><div class="div_a1-keleyi-com"><a href="http://keleyi.com/menu/net/">新一代微软平台编程语言</a></div></li>
<li><a href="http://keleyi.com/menu/javascript/">JAVA</a><div class="div_a1-keleyi-com"><a href="http://keleyi.com/a/bjac/9a9irmcm.htm">最有前途的编程语言</a></div></li>
<li><a href="http://keleyi.com/a/bjac/ck9atu5j.htm">DELPHI</a><div class="div_a1-keleyi-com"><a href="http://keleyi.com/a/bjac/5939d3b2c920ff6d.htm">最具潜力的编程语言</a></div></li>
</ul>
</div><!--柯 乐 义-->
</body>
</html>