.net中div置于顶层+iframe

aspx代码：

<td>
  <asp:Button ID="BtnDownPPT" runat="server" OnClientClick="ShowPPT();" Text=" 公报下载 "
 Visible="false" />&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
 </td>

…………

<asp:Button ID="Btn_DownPPT" runat="server" Text=" 公报下载 " CssClass="displayNone"
                    OnClick="Btn_DownPPT_Click" />

…………

<div id='pageBase_Div' style='display: none; z-index: 12001; text-align: center;
        font-size: large; vertical-align: middle; position: absolute; left: 400px; top: 200px;
        padding: 5px 6px 0px 6px; border: 1px solid blue;' runat="server">
        <table>
            <tr>
                <td>
                    <img alt="请稍候......" src="../../App_Themes/Green/Images/loading.gif" />
                </td>
                <td valign="middle">
                    正在下载报表，请稍候......
                </td>
            </tr>
        </table>
    </div>
    <iframe id="pageBase_iframe" src="" scrolling="no" frameborder="0" style="position: absolute;
        top: 200px; left: 400px; display: none;" runat="server"></iframe>

js:

function showDownPPT() {
            //            debugger;
            $("#Btn_DownPPT").click();
            document.getElementById('divContent').style.display = 'none';
            var nums = document.getElementById("div_QueryCon");
            var buttons = nums.getElementsByTagName("input");
            for (var i = 0; i < buttons.length; i++) {
                buttons[i].style.color = "#000000"; //将按钮字体变黑
                buttons[i].disabled = true;

//所有按钮都置为不可用，所以不能再按钮中加入OnClick="Btn_DownPPT_Click"事件，所以要先触发 $("#Btn_DownPPT").click();
            }
            document.getElementById('BtnDownPPT').style.color = "#FF0000"; //下载按钮字体变红色
            var pagebase_div = document.getElementById('pageBase_Div');
            var pagebase_iframe = document.getElementById('pageBase_iframe');
            pagebase_div.style.display = "block";
            pagebase_iframe.style.width = pagebase_div.offsetWidth;
            pagebase_iframe.style.height = pagebase_div.offsetHeight;
            pagebase_iframe.style.top = pagebase_div.style.top;
            pagebase_iframe.style.left = pagebase_div.style.left;
            pagebase_iframe.style.zIndex = pagebase_div.style.zIndex - 1;
            pagebase_iframe.style.display = "block";
            setTimeout("HideDownPPT()", 70000); //默认70秒
        }
        function HideDownPPT() {
            //            debugger;
            var pagebase_div = document.getElementById('pageBase_Div');
            var pagebase_iframe = document.getElementById('pageBase_iframe');
            pagebase_div.style.display = "none";
            pagebase_iframe.style.display = "none";
            var nums = document.getElementById("div_QueryCon");
            var buttons = nums.getElementsByTagName("input");
            for (var i = 0; i < buttons.length; i++) {
                buttons[i].style.color = "#000000"; //将按钮字体变黑
                buttons[i].disabled = false;
            }
            document.getElementById('divContent').style.display = 'block';
        }