<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<style>
*{ margin:0; padding:0; list-style:none;}
img{ border:0;}
.header{ width:100%; background:#F5F5F5;}
.nav{ width:1000px; margin:0 auto; overflow:hidden;}
.nav ul li{ height:40px; line-height:40px; float:left; padding:10px 5px; margin:0px 5px;position:relative;}
.nav ul li a{ color:#666; font-family:‘Microsoft Yahei‘; font-size:14px; text-decoration:none;}
.nav ul li a:hover{ color:#000; text-decoration:none;}
.nav ul li span{ display:block; position:absolute; width:0px; height:0px; background:#1FAEFF; top:58px; left:50%;}
</style>
</head>
<body>
<div class="header">
<div class="nav">
<ul>
<li><a>首页</a><span></span></li>
<li><a>菜单导航</a><span></span></li>
<li><a>时间日期</a><span></span></li>
<li><a>焦点图</a><span></span></li>
<li><a>tab标签</a><span></span></li>
<li><a>jquery特效</a><span></span></li>
<li><a>相册代码</a><span></span></li>
<li><a>图片特效</a><span></span></li>
<li><a>名站特效</a><span></span></li>
</ul>
</div>
</div>
<script src="http://apps.bdimg.com/libs/jquery/2.1.1/jquery.min.js"></script>
<script>
$(function(){
$(‘.nav li‘).hover(function(){
$(‘span‘,this).stop().css(‘height‘,‘2px‘);
$(‘span‘,this).animate({
left:‘0‘,
width:‘100%‘,
right:‘0‘
},200);
},function(){
$(‘span‘,this).stop().animate({
left:‘50%‘,
width:‘0‘
},200);
});
});
</script>
</body>
</html>
原文：http://www.cnblogs.com/ws3366/p/5175510.html