不设置botton，就不显示a标签？

<!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" xml:lang="en">
<head>
    <title>Javascript 回到顶部效果</title>
   
   <style>
       .box{
               width:1190px;
               margin:0 auto;
       }
       #btn{
           position:fixed;
           width:40px;
           height:40px;
           left:50%;
           /*bottom:30px;*/
           margin-left: 610px;
           background:url(images/top_bg.jpg) no-repeat left top;
       }

   </style>
</head>

<body>
   <div>
       <img src="images/tb_bg.jpg" />
   </div>
   <a href="javascript:;" id="btn" title="回到顶部"></a>
</body>
</html>