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>放大啦 " ;
        }
         if (window.screen.width > _screenOriginalWidth){
            document.getElementById( " output " ).innerHTML = " <br>缩小啦 " ;
        }
        _screenOriginalWidth = window.screen.width
    }
})()
// -->
</ SCRIPT >
</ BODY >
</ HTML >