如何在中心打开弹出窗口？

Question

我正在尝试打开我在中心的弹出窗口。我的页面有滚动条。我已经写了css 我的弹出窗口打开一个固定的位置，但我想在中心打开我的弹出窗口意味着如果我在按钮上，那时候我的弹出窗口也会在中间打开。然后弹出窗口打开但任何位置

<div id="editAssessmentFormId:popupIdShow"
    style="visibility: hidden; display: none; position: absolute; top: 200px; left: 450px; border: none; overflow-x: auto; width: 350; no-repeat top left; align: center;">
    <table width="350" border="0" cellspacing="0" cellpadding="0">
        <tr>
            <td align="left" valign="top">
                <div>
                    <table border="0" cellspacing="0" cellpadding="0" width="100%">
                        <tr>
                            <td align="left" valign="top" width="5"><img
                                src="../images/popup_topleft.gif" width="5" height="27" /></td>
                            <td height="27" align="left" valign="middle" class="bg5">
                                <table width="100%" border="0" cellspacing="0" cellpadding="0">
                                    <tr>
                                        <td width="96%" align="left" valign="middle"
                                            class="popupTopPanel"><strong>Upload Image</strong></td>
                                        <td width="4%" align="right" valign="middle"><img
                                            src="../images/btn_close.gif" width="19" height="19"
                                            onclick="return closeThePopup();"
                                            onmouseover="this.className='flyOver'"
                                            onmouseout="this.className='flyOut'" /></td>
                                    </tr>
                                </table>
                            </td>
                            <td align="right" valign="top" width="5"><img
                                src="../images/popup_topright.gif" width="5" height="27" /></td>
                        </tr>
                    </table>
                </div>
                <div
                    style="background-color: #FFFFFF; border: 1px solid; border-color: #ffffff #7D9BBA #ffffff #7D9BBA;">
                    <table border="0" cellspacing="0" cellpadding="0" width="90%"
                        style="margin: 0 0 0 10px;">
                        <tr>
                            <td align="center" valign="middle" height="30"><t:inputFileUpload
                                    id="questionFileId" value="#{launchPlayer.fileName}"
                                    onfocus="window.clipboardData.clearData();"
                                    onkeypress="return false;" storage="file" /></td>
                        </tr>
                        <tr>
                            <td align="center" valign="middle" height="30"><t:commandButton
                                    image="../images/btn_submit.gif"
                                    action="#{launchPlayer.uploadImage}"
                                    onclick="return hideThePopup();"></t:commandButton></td>
                        </tr>
                    </table>
                </div>
                <div>
                    <table border="0" cellspacing="0" cellpadding="0" width="100%">
                        <tr>
                            <td align="left" valign="top" width="6"><img
                                src="../images/bottomleft.gif" width="6" height="8" /></td>
                            <td align="left" valign="middle" bgcolor="#FFFFFF"
                                style="border-bottom: 1px solid #7D9BBA;"><img
                                src="../images/spacer.gif" width="1" height="1" /></td>
                            <td align="right" valign="top" width="6"><img
                                src="../images/bottomright.gif" width="6" height="8" /></td>
                        </tr>
                    </table>
                </div>
            </td>
        </tr>
    </table>
</div>

这是我的代码，如果我错了请纠正