<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
div{
position: relative;
}
p{
width: 50px;
height: 50px;
background: red;
opacity: 0.5;
position: absolute;
z-index: 999;
top: 0;
left: 0;
}
</style>
</head>
<body>
<h3>外部元素</h3>
<h3>外部元素</h3>
<h3>外部元素</h3>
<div>
<h3>内部元素</h3>
<h3>内部元素</h3>
<h3>内部元素</h3>
<p></p>
</div>
</body>
</html>
原文：http://blog.51cto.com/12173069/2113281