判断IE浏览器用IE条件表达式

<!--[if IE]> 
   <script type="text/javascript"> 
      alert("ie") 
   </script> 
< ![endif]--> 


<!--[if IE 6]> 
    <script type="text/javascript"> 
       alert("ie6") 
    </script> 
< ![endif]--> 


<!--[if IE 7]> 
     <script type="text/javascript"> 
         alert("ie7") 
     </script> 
< ![endif]-->