I have tried this test case and for me validation works fine. It's show error message that start tag h:inputText not closed
Weird - I just tried and the same thing is happening for (i.e. I get no validation error)
It seem to only happen for namespaced tags.
i.e. <p style="xyz /> gives me a warning/error.
<h:inputText readonly="true /> does not.
kphilipp, please report this as a bug - we need to investigate that but looks like we have to live with that bug for CR1 ;(