div悬浮

<!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=utf-8" />
<title></title>
<style type="text/css">
</style>
</head>
<body>
<!--z-index 属性设置元素的堆叠顺序。拥有更高堆叠顺序的元素总是会处于堆叠顺序较低的元素的前面。z-index 仅能在定位元素上奏效（例如 position:absolute;）！-->
<div style='position:relative; 500px; height:500px;'>
<div style='position:absolute; z-index:2; 300px; height:300px; left:0px; top:0px; background-color:red;'></div>
<div style='position:absolute; z-index:3; 200px; height:200px; left:0px; top:0px;background-color:green;''></div>
</div>
</body>
</html>
效果：