Ops, the first patch was introducing a bug in the nevow:invisible rendering, please ignore it and use the new one 
Sorry about that, ciao ste
Hello, I just submitted a couple of patches to the Nevow issue tracker.
The first one  is to make flatsax.ToStan use the namespace prefix of elements that are not in the default namespace. I also added a new testcase for it.
The patch also fixes an already existing flatsax testcase which is now in TODO mode (test_switchns), at least as far as correct XML namespace usage is concerned, even if it would still fail because the input and output strings are not exactly the same. That testcase is probably superfluous if the new one gets checked in.
The second one  is a trivial change to the default 404 page so that it is wellformed XML.
Thanks, ciao ste