表单美化的js代码

<!DOCTYPE html>
<html>
   <head>
       <meta charset="UTF-8">
       <title>单选按钮美化</title>
       <style type="text/css">
           *{
               margin: 0;
               padding: 0;
           }
           #content{
               width: 400px;
               height: 32px;
               border: 1px solid #DFDFDF;
               margin: 50px auto 0;
               font-size: 12px;
               font-family: "微软雅黑";
           }
           #radio{
               height: 32px;
               line-height: 32px;
               padding-left: 15px;
           }
           #radio dt{
               float: left;
           }
           #radio dd{
               float: left;
               margin: 0 10px 0 8px;
               padding-left: 23px;
               position: relative;
           }
           #radio dd a{
               text-decoration: none;
               color: #000000;
           }
           #radio dd a:hover{
               text-decoration: underline;
               color: #CC0000;
           }
           #radio dd a:hover b{
               background-position:-15px -118px;
           }
           #radio dd b{
               display: block;
               width: 20px;
               height: 20px;
               background: url(images/radiobutton.gif) no-repeat -15px -18px;
               position: absolute;
               top:8px;
               left: 0;
           }
           #radio .selected b,#radio .selected a:hover b{
               background-position:-15px -218px;
           }
       </style>
   </head>
   <body>
       <div id="content">
           <dl id="radio">
               <dt>配送类型:</dt>
               <dd class="selected"><a href="#"><b></b>全部</a></dd>
               <dd><a href="#"><b></b>京东</a></dd>
               <dd><a href="#"><b></b>第三方</a></dd>
           </dl>
       </div>
       <script type="text/javascript">
           window.onload=function(){
               var oas=document.getElementsByTagName("a");
               //alert(oas.length);
               for(var i=0;i<oas.length;i++){
                   oas[i].onclick=function(){
                       for(var j=0;j<oas.length;j++){
                           oas[j].parentNode.className="";
                       }
                       oas[this].parentNode.className="selected";
                   }
               }
           }
       </script>
   </body>
</html>

我看到评论说这个思路与Tab一样，所以我就按照那个思路写的可是浏览器中最后一句报错，这种思路不可以吗？求大神解释