急求，一个关于JS兼容FF浏览器的有关问题 _JavaScript

急求，一个关于JS兼容FF浏览器的问题
JS代码如下：

var xPos = 300;
var yPos = 200;
var step = 1;
var delay = 30;
var height = 0;
var Hoffset = 0;
var Woffset = 0;
var yon = 0;
var xon = 0;
var pause = true;
var interval;
img1.style.top = yPos;
function changePos()
{
width = document.body.clientWidth;
height = document.body.clientHeight;
Hoffset = img1.offsetHeight;
Woffset = img1.offsetWidth;
img1.style.left = xPos + document.body.scrollLeft;
img1.style.top = yPos + document.body.scrollTop;
if (yon)
{yPos = yPos + step;}
else
{yPos = yPos - step;}
if (yPos < 0)
{yon = 1;yPos = 0;}
if (yPos >= (height - Hoffset))
{yon = 0;yPos = (height - Hoffset);}
if (xon)
{xPos = xPos + step;}
else
{xPos = xPos - step;}
if (xPos < 0)
{xon = 1;xPos = 0;}
if (xPos >= (width - Woffset))
{xon = 0;xPos = (width - Woffset); }
}

function start()
{
img1.visibility = "visible";
interval = setInterval('changePos()', delay);
}
function pause_resume()
{
if(pause)
{
clearInterval(interval);
pause = false;}
else
{
interval = setInterval('changePos()',delay);
pause = true;
}
}
start();

HTML代码如下：
<!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">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
</head>

<body style=" width:100%; height:1000px;">
<DIV id=img1 style="Z-INDEX: 100; LEFT: 2px; WIDTH: 59px; POSITION: absolute; TOP: 43px; HEIGHT: 61px;
visibility: visible;"><a href="http://www.makewing.com/lanren" target="_blank"><img src="images/pic.gif" width="80" height="80" border="0"></a></DIV>
<SCRIPT src="js/js.js"></SCRIPT>
</body>
</html>

上面这段html代码，如果把<!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">
去掉，就能兼容FF，我百度了一下说是JS代码不规范的原因，请大神帮帮忙，怎么修改才能把JS修改规范呢。这段JS的效果是制作一个网页上的漂浮广告窗口。在线等，急求！！！

------解决方案--------------------
<!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">加了这个申明，设置left，top位置或者大小之类的属性时，需要加上单位。。

JScript code

var xPos = 300;
var yPos = 200; 
var step = 1;
var delay = 30; 
var height = 0;
var Hoffset = 0;
var Woffset = 0;
var yon = 0;
var xon = 0;
var pause = true;
var interval;
img1.style.top = yPos+'px';//////
function changePos() 
{
width = document.body.clientWidth;
height = document.body.clientHeight;
Hoffset = img1.offsetHeight;
Woffset = img1.offsetWidth;
img1.style.left = xPos + document.body.scrollLeft+'px';//////
img1.style.top = yPos + document.body.scrollTop+'px';//////