求自动跟随DIV js
像淘宝里 商品详情 那条选项卡超过了高度都自动跟随的代码


效果如图
效果地址：http://detail.tmall.com/item.htm?spm=a2106.m895.1000384.d11.Jjhd6L&id=20503104378&source=dou&scm=1029.newlist-0.bts7.50011131&ppath=

谢谢
------解决方案--------------------

<style type="text/css">

    #scs{

        border: 1px solid red;

        height:20px;

        font:12px/25px "宋体";

        width:500px;

    }

</style>

<div style="height:200px"></div>

<div id="scs">这是固定的</div>

<div style="height:2000px"></div>

<script type="text/javascript">

    window.onload=function(){

        var scs=document.getElementById("scs");//获取对象

        var t=scs.offsetTop;//获得本身到网页顶部的距离

        var isIE6 = !-[1,] && !window.XMLHttpRequest;//判断ie6

        var i=0;

        window.onscroll=function(){

            i=document.documentElement.scrollTop;

            if(i>t){

                //网页被卷去的高大于本身到网页顶部的距离（简单理解就是对象开始要被卷到网页顶部不可视区域）

                if(isIE6){

                    scs.style.position="absolute";

                    scs.style.top=i+"px";

                }else{

                    scs.style.position="fixed";

                    scs.style.top=0;

                }

            }else{

                scs.style.position="static";

                scs.style.top="auto";

            }

        }

    }

</script>