function getTop(e)
{
    var offset=e.offsetTop;
    if(e.offsetParent!=null) //只要還有父元素,也就是當前元素不是根節點就繼續往上累計元素的高度
        offset+=getTop(e.offsetParent);
    return offset;
}
var myBlockTop = getTop(document.getElementById("homepageBanner"));
var oneDiv=document.getElementById("homepageBanner");
if(!!window.attachEvent)//
 
ie瀏覽器下。
{
    window.attachEvent('onscroll',function(){
                    if(document.documentElement.scrollTop/* +  (document.body.clientHeight || window.innerHTML) */>= myBlockTop)
                    {oneDiv.style.position="fixed";oneDiv.style.top="0";oneDiv.style.left="50%";oneDiv.style.marginLeft="-500px";}
                    
 
else{oneDiv.removeAttribute("style");}

    });
}
if(!!window.addEventListener)//非ie瀏覽器下
{
    window.addEventListener("scroll",function(){//document.body.scrollTop可保證chrome的正常。
        if(document.documentElement.scrollTop/* +  (document.body.clientHeight || window.innerHTML) */>= myBlockTop||document.body.scrollTop>=myBlockTop)
        {oneDiv.style.position
 
="fixed";oneDiv.style.top="0";oneDiv.style.left="50%";oneDiv.style.marginLeft="-500px";}
        else{oneDiv.style.position="";oneDiv.style.top="auto";oneDiv.style.left="auto";oneDiv.style.marginLeft="0";oneDiv.removeAttribute("style");}
    });
}