怎么让一个div 悬浮在另一个div上
<!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>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title></title>
<style type="text/css">
<!--
body{
font-size:12px;
}
.box{
width:500px;
}
.c12{
color:#f60;
padding:0.5em 0;
}
.serverDetail{
width:215px;
height:75px;
position:relative;
margin-top:-5px;
margin-left:-5px;
background:#FAFAFA;
border:1px solid #ccc;}
.shadow{
width:215px;
height:75px;
float:left;
margin:7px 0 7px 18px !important;
margin:7px 5px 7px 10px;
background:#E2DEDE;}
.serverDetail div{
padding-left:12px;}
-->
</style>
</head>
<body>
<div class="box">
<div class="shadow">
<div class="serverDetail">
<div class="c12"><strong>短信服务</strong></div>
<div>一次点击无限商机，在最需要的时候找到你</div>
</div>
</div>
<div class="shadow">
<div class="serverDetail">
<div class="c12"><strong>短信服务</strong></div>
<div>一次点击无限商机，在最需要的时候找到你</div>
</div>
</div>
<div class="shadow">
<div class="serverDetail">
<div class="c12"><strong>短信服务</strong></div>
<div>一次点击无限商机，在最需要的时候找到你</div>
</div>
</div>
<div class="shadow">
<div class="serverDetail">
<div class="c12"><strong>短信服务</strong></div>
<div>一次点击无限商机，在最需要的时候找到你</div>
</div>
</div>
<div style="clear:both;"></div>
</div>
</body>
</html>
