RequiredFieldValidator not working

With the following simple mark-up, I get very strange behaviour in FF and IE8. If I give the textbox focus, and tab out, nothing happens. If I give a user name value, and erase it immediately开发者_开发知识库, nothing happens. However, only when I supply a user name, tab away, the erase it and tab away again, do I finally get a red star "required" mark. The summary doesn't show at all.

This is the markup I was trying with. Looks like my issue was with EnableClientScript and ValidationGroup:

<asp:Label ID="userNameLabel" runat="server"
           AssociatedControlID="userNameText">
           User Name:
</asp:Label>
<asp:TextBox ID="userNameText" runat="server" 
             Width="200px">
</asp:TextBox>
<asp:RequiredFieldValidator ID="userNameRequired" runat="server" 
                            ControlToValidate="userNameText" 
                            Display="Dynamic" 
                            EnableClientScript="true" 
                            ValidationGroup="userValidation"
                            ErrorMessage="User Name is always required.">
           *   
</asp:RequiredFieldValidator>

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Are you sure EnableClientScript="true" is even needed? I think it defaults to that.