网页：鼠标悬停导航栏下划线动画效果

.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:50px; left:50%;}

<script>
$(function(){
	$('.nav li').hover(function(){
		$('span',this).stop().css('height','8px');
		$('span',this).animate({
			left:'0',
			width:'100%',
			right:'0'
		},200);
	},function(){
		$('span',this).stop().animate({
			left:'50%',
			width:'0'
		},200);
	});
});
</script>

<div class="header">
<div class="nav">
	<ul>
	<li><a href='#'>首页</a><span></span></li>
    	<li><a href='#'>标题1</a><span></span></li>
    	<li><a href='#'>标题2</a><span></span></li>
    	<li><a href='#'>标题3</a><span></span></li>
    	<li><a href='#'>标题4</a><span></span></li>
    	<li><a href='#'>标题5</a><span></span></li>
    	<li><a href='#'>标题6</a><span></span></li>
    	<li><a href='#'>标题7</a><span></span></li>
    	<li><a href='#'>标题8</a><span></span></li>
    	<li><a href='#'>标题9</a><span></span></li>
    	<li><a href='#'>标题10</a><span></span></li>
    	<li><a href='#'>标题11</a><span></span></li>
    	<li><a href='#'>标题12</a><span></span></li>
	</ul>
</div>
</div>

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