"Client-side" validation not working for custom server control in IE

[Ron]

I've an aspx page with a custom (composite) server control and a
TextBox, both with RequiredFieldValidators. I'd like the custom
control to behave the same way the default TextBox does when one
clicks the submit button - a trip to the server is *not* made when
using Internet Explorer. Validation works for the custom control, but
only after a trip to the server.

Does anyone know why this is the case and if there is a way to force
validation to fire for the custom control without a trip to the
server?

TIA,

Ron
(e-mail address removed)