怎么知道页面被放大缩小了

有个投诉说页面变形了，一问才知道页面被用户不小心zoom了，这个情况最好是能对用户有个提示。写了个IE和firefox下检测zoom的demo：

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" > 
<HTML > 
<BODY > 
<div  id ="output" ></div > 
<SCRIPT  LANGUAGE ="JavaScript" > 
( function (){ 
    window.onresize = testZoom; 
     var  _screenOriginalWidth = window.screen.width 
     function  testZoom(){ 
         if (window.screen.width < _screenOriginalWidth){ 
            document.getElementById("output").innerHTML = " <br>Zoom In " ; 
        } 
         if (window.screen.width > _screenOriginalWidth){ 
            document.getElementById("output").innerHTML = " <br>Zoom Out " ; 
        } 
        _screenOriginalWidth = window.screen.width 
    } 
})()
</SCRIPT> 
</BODY> 
</HTML>