AJAX 中，讓 UpdateProgress 中的內容顯示在頁面正中央

<asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="UpdatePanel1">
        <ProgressTemplate>
           
            <div style="position:absolute;left:50%;top:50%;">
               <table width="214" align="center">
               <tr>
               <td>正在載入資料,請稍候</td>
                <td><img src="images/loading_16x16.gif" alt="請稍候"/></td>
               </tr>
               <tr>
               <td colspan="2"><img src="images/pleasewait.gif" /></td>     
               </tr>
               </table>
            </div>
            

        </ProgressTemplate>
    </asp:UpdateProgress>

關鍵是這句話：

<div style="position:absolute;left:50%;top:50%;">