Why does ModalPopupExtender not work when I move its target panel's style from hardcoded to CSS?

Question

I have a ModalPopupExtender set up on a page and working great. It's tied to a panel with some style attributes hardcoded into its tag. I'm trying to get all my styles out of my markup into CSS, but when I move these styles into a class, the popup fails to work. What's going on?

Styles in CSS:

.class1
{
    border: solid 1px black;
    display: none;
    width: 700px;
    height: 400px;
    background-color: gray;
    overflow: scroll;
}
.class2
{
    background-color: White;
    height: 90%;
    width: 95%;
}
.backgroundClass
{
    background-color: gray;
    filter: alpha(opacity=70);
    opacity: 0.7;
}

Panel:

<asp:Panel ID="pnlModalContainer" runat="server"  CssClass="class1">
    <asp:Panel ID="pnlModalHandle" runat="server">
        <table cellpadding="0" cellspacing="0" border="0" width="650px">
            <tr>
                <td>
                    Content Here&nbsp;|&nbsp;
                    <asp:Label ID="lblTitle" runat="server" Text=""></asp:Label>
                </td>
            </tr>
        </table>
    </asp:Panel>
    <div style="padding-left: 25px;">
        <table cellpadding="1" cellspacing="0" border="0" class="class2">
            <tr valign="top">
                <td>
                    <asp:UpdatePanel ID="udpPopups" runat="server">
                        <ContentTemplate>
                            ...content here...
                        </ContentTemplate>
                    </asp:UpdatePanel>
                </td>
            </tr>
            <tr valign="bottom">
                <td align="right" style="padding-bottom: 5px; padding-right: 5px;">
                    <asp:Button ID="btnModalOk" runat="server" Text="Ok" />&nbsp;&nbsp;
                    <asp:Button ID="btnModalCancel" runat="server" Text="Cancel" CausesValidation="false" />
                </td>
            </tr>
        </table>
    </div>
</asp:Panel>

And the MPE:

<cc1:ModalPopupExtender ID="mpeAction" runat="server" PopupControlID="pnlModalContainer"
TargetControlID="fakeButton" BackgroundCssClass="backgroundClass" DropShadow="false"
PopupDragHandleControlID="pnlModalHandle" RepositionMode="RepositionOnWindowResizeAndScroll" />

This is in ASP .NET 2.0. Set up like this, I see the background show up, but the popup doesn't come up. But if I move the contents of class1 down to the markup as a style, it works great. The popup gets shown via Javascript emitted from VB code-behind, using ScriptManager.RegisterStartupScript().