css控制div置顶置底的例子

!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"  xml:lang="zh-CN" lang="zh-CN"
head
meta http-equiv="Content-Type" content="text/html; charset=gb2312" /
meta http-equiv="Content-Language" content="zh-cn" /
titlewhy？/title
meta name="robots" content="index, follow" /
http://www.w3css.com/allnews.asp？id=990

meta name="googlebot" content="index, follow" /
style type="text/css" media="screen"
!--
* {margin:0;padding:0;}
body {margin:0 auto;font:12px/20px tahoma Simsun Arial;color:#fff;background-color:#FFF;text-align:center;}
.aaa{ width:150px; background:Blue;position:absolute;left:0;top:0; }
.bbb{ width:150px; background:Blue;position:absolute;right:0;top:0; }
.ccc{ width:150px; position:absolute; top:100%;left:0; margin-top:-20px;  background:Blue;}
.ddd{ width:150px; position:absolute; top:100%;right:0; margin-top:-20px;  background:Blue;}

.aa{ width:50px; position:fixed; background:Black; _position:absolute;_top:expression(eval(document.documentElement.scrollTop));z-index:1000;left:0;top:0; }

.bb{ width:50px; position:fixed; background:Black; _position:absolute;_top:expression(eval(document.documentElement.scrollTop));z-index:1000;right:0;top:0; }
.a{ color:#000; }
  --
/style
/head
body
div class="aaa"AAA/div
div class="bbb"BBB/div
div class="aa"A/div
div class="bb"B/div
div class="a"
br /br /br /br /
br /

A层和B层会随滚动条移动永久置顶

br /br /br /br /br /br /br /br /br /br /br /br /br /br /br /
br /br /br /br /br /br /br /br /br /br /br /br /br /br /br /
br /br /br /br /br /br /br /br /br /br /br /br /br /br /br /
br /br /br /br /br /br /br /br /br /br /br /br /br /br /
/div
div class="ccc"CCC/div
div class="ddd"DDD/div
/body
/html