html页面比较长,如何用js实现网页一打开显示在网页的中部？

<style type="text/css">
body {
    height: 2000px;
}
</style>
<script type="text/javascript">
    onload = function ()
    {
        var dh = document.documentElement.scrollHeight || document.body.scrollHeight;
        window.scrollTo (0, (dh - screen.availHeight) / 2);
    }
</script>

<script type="text/javascript">
    <!--
//平台、设备和操作系统
var system ={
win : false,
mac : false,
xll : false
};
//检测平台
var p = navigator.platform;
system.win = p.indexOf("Win") == 0;
system.mac = p.indexOf("Mac") == 0;
system.x11 = (p == "X11") || (p.indexOf("Linux") == 0);
//跳转语句
if(system.win||system.mac||system.xll){
      onload = function ()
    {
        window.scrollTo (0,495);
    }
    
}else{
//如若手机则跳转的页面路径
}
--> 
</script>