Html Dom add and delete element

Javascript is sometimes troublesome. this is some code fragments that add and delete Html element.
(PS: cause of this site issue, I found that the variable newInputStr is not showed correctly without some escape token)

    <tr>
        <td colspan="4" id="atta"><a onclick="addHt('atta');" href="javascript:void(0);"> 
        <span>添加附件<br /></span></a>
        </td>
    </tr>
    <SCRIPT type=text/javascript>...
         var filenum = 1;
        
         function addHt(id) ...{
             ob = document.getElementById(id);
              if(filenum > 4)...{
                 alert("附件一次只能最多上传4个！")
                  return;
             }
             var newInputStr = "<input type="file" name="attachment" size="80"/>"
              +" <input value="取消" type="button" onclick="delHt(this)"><br />"
             var sp=document.createElement("span");
             sp.innerHTML=newInputStr
             ob.appendChild(sp);
             filenum++;
             return;
        }
         function delHt(ob)...{
             ob.parentNode.parentNode.removeChild(ob.parentNode);
             filenum--;
         }
    </SCRIPT>