<!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 runat="server">
<title></title>
<style type="text/css">
body
{
background-color:Gray;
}
#top
{
background-color: Yellow;
height: 100px;
position: absolute;
top:100px;
width:200px;
}
#a1
{
border-color: transparent transparent Yellow transparent;
border-style: solid;
border-width: 20px;
position: absolute;
margin-left: -6px;
left: 50%;
top: -40px;
}
</style>
</head>
<body>
<form id="form1" runat="server">
<div id="top">
top
<div id="a1">
</div>
</div>
</form>
</body>
</html>
原文：http://www.cnblogs.com/zhwl/p/4337586.html