We use a MVVM model for our UIs where our "views" (pure xaml) have no code in the codebehind and use databinding to connect to viewmodels, which provide data and respond to events triggered by the views. We do not put unit tests around the xaml (though this is possible in Silverlight), but it is easy to put unit tests around the viewmodel code. That is, "when x button is pressed xyz request must be sent to the server" is a fairly trivial test to write.

I have not written much javascript, but I would think that tools exist to make similar techniques work with well designed HTML/JS UIs as well.

Additionally the UI team is responsible for writing the server side code that sits behind web services and performs all of the CRUD logic, including validation. This code is easily testable.

So there is quite a lot of testing that can be performed, though obviously they'll never completely match the test coverage rates that we do.