<html>
<head></head>
<body>
<div style="height:40%;width:40%;background-color:#999;z-index:2;position:absolute;">aaa</div>
<div style="height:40%;width:40%;background-color:#0CF;position:absolute;left:20%;top:20%;z-index:1">bbb</div>
</body>
</html>
示例如下图：
