FireFox里设置top left没作用

<!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=utf-8" />
<title>无标题文档</title>
</head><body><script language="javascript"> 
function a(){ 
window.document.getElementById("test1").style.top= "300px"; 
window.document.getElementById("test1").style.left= "200px"; 
} 
</script> 
<body> 
<input type="button" onclick="a();" value="sdf" /> 
<iframe id="test1" name="test1" style="height:200px; width:200px; position:absolute;z-index:100"> </iframe> 
</body>
</html>