使用css实现缩进带背景无限嵌套，支持Ie6，代码如下：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
*{margin:0;padding:0;}
li{list-style:none;}
.box{300px;height:400px;border:2px solid #ccc;margin:10px;position:relative;}
.box li{padding-left:15px;line-height:20px;}
.box a{display:block;}
.box .bg{position:absolute;z-index:-1;left:0;300px;height:20px;background:#ccc;opacity:0;filter:alpha(opacity=0);}
.box a:hover .bg{opacity:1;filter:alpha(opacity=100);}
</style>
</head>
<body>
<div class="box">
<ul>
<li>
<a href="#"><span class="bg"></span>12</a>
<ul>
<li>
<a href="#"><span class="bg"></span>12</a>
<ul>
<li>
<a href="#"><span class="bg"></span>12</a>
<ul>
<li><a href="#"><span class="bg"></span>12</a></li>
<li><a href="#"><span class="bg"></span>12</a></li>
</ul>
</li>
<li><a href="#"><span class="bg"></span>12</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</div>
</body>
</html>