层固定方法（position:fixed）
<!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>
<style>
*{margin:0}
body{
height:1000px;
width:1000px;
}
#a{
width:300px;
height:100px;
background:red;
}
</style>
<script>
var isie6 = window.XMLHttpRequest?false:true;
window.onload = function(){
var a = document.getElementById('a');
var d = document.getElementById('d');
if(isie6){
a.style.position = 'absolute';
window.onscroll = function(){
d.innerHTML = '';
}
}else{
a.style.position = 'fixed';
}
a.style.right = '0';
a.style.bottom = '0';
}
</script>
</head>
<body>
<div id ="d" style="display:none;"></div>
<div id='a'>test</div>
</body>
</html>