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效果图，如上图所示
<!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>
<title></title>
<style type="text/css">
#menu
{
margin-left:auto;
margin-right:auto;
width:1000px;
}
ul {
margin: 0px;
padding: 0px;
height:30px;
}
ul li {
float: left;
display: inline;
font: 0.9em Arial, Helvetica, sans-serif;
height: 30px;
width: 100px;
list-style: none;
}
ul li a {
color: #F4F4F4;
text-decoration: none;
text-align:center;
line-height: 29px;
width: 91px;
margin: 0px;
padding: 0px 0px 0px 8px;
display: block;
border-right: solid 1px #ccc;
border-bottom:solid 1px #ccc;
background: #808080;
}
ul li ul li { height:25px; }
ul li ul li a {
background: #666;
line-height:24px;
}
ul li a:hover { background: #666; }
ul li ul { visibility: hidden; }
ul li:hover ul { visibility: visible; }
ul li ul li a:hover { background: #333; }
</style>
</head>
<body>
<div id="menu">
<ul>
<li><a href="#">菜单一</a></li>
<li><a href="#">菜单二</a>
<ul>
<li><a href="#">子菜单一</a></li>
<li><a href="#">子菜单二</a></li>
<li><a href="#">子菜单三</a></li>
</ul>
</li>
<li><a href="#">菜单三</a></li>
<li><a href="#">菜单四</a>
<ul>
<li><a href="#">子菜单一</a></li>
<li><a href="#">子菜单二</a></li>
<li><a href="#">子菜单三</a></li>
</ul>
</li>
<li><a href="#">菜单五</a></li>
</ul>
</div>
</body>
</html>