文本框内容替空时 边框变为红色。js控制文本框的class

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
     <title>
     </title>
     <!--设置文本框的边框颜色和样式-->
     <style type="text/css">
       .input{
	   border-color:#FF0000;
	   border-style:solid;
	   }
     </style>
     <script type="text/javascript" language="javascript" >
	 <!--当文本框为空时，加载css的方法-->
        function fun(test){
		  var value= document.getElementById(test).value;
		  var object= document.getElementById(test);
		  if(value==null||value==''){
		    object.setAttribute("className","input");
		  }else{
		    object.setAttribute("className","");
		  }
		    
		}
     </script>
  </head>
  <body>
    <input type="text" onBlur="fun('user_name')" id="user_name" >  
  </body>
</html>

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