<!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>无标题文档</title>
<style>
*{margin:0; padding:0;}
.fixed{position:fixed; top:0; left:0; width:500px;height:100px; background:#f60; margin-left:100px;}
</style>
</head>
<body>
<div style="height:300px;"></div>
<div id="test" style="height:100px; background:#f60; margin-left:100px;"></div>
<script>
var test=document.getElementById("test");
var X= test.getBoundingClientRect().top;
window.onscroll=function(){
if(document.documentElement.scrollTop>=X||document.body.scrollTop>=X){
test.className="fixed";
}else{
test.className="";
}
};
/*
var X= this.getBoundingClientRect().left;
var Y =this.getBoundingClientRect().top;
再加上滚动距离，就可以得到绝对位置
var X= this.getBoundingClientRect().left+document.documentElement.scrollLeft;
var Y =this.getBoundingClientRect().top+document.documentElement.scrollTop;
*/
</script>
<div style="height:1000px;"></div>
</body>
</html>
原文：http://www.cnblogs.com/dingyuanxin/p/3918350.html