div随滚动条上下滚动，onscroll事件兼容IE、火狐、谷歌浏览器

<script type="text/javascript">
var scrollFunction=function(e){
e=e || window.event;
var _h = document.documentElement.clientHeight;
var _s = document.documentElement.scrollTop||document.body.scrollTop;
var _m = _s + (_h - 500) * 0.5;
if (_m < 0) {
_m = 0;
}
document.getElementById("divid").style.top=_m+"px";
};
//FireFox
if(document.addEventListener){
document.addEventListener("DOMMouseScroll" ,scrollFunction, false);
}
//IE、Opera、Chrome
window.onscroll=document.onscroll=scrollFunction;
</script>