Check for well-formed XML and for proper DOCTYPE declaration is broken for docbooks in kf5 using dtd/kdedbx45.dtd, see e.g.
There still seem to be issues. DOCTYPE says the catalog wants V4.2, not V4.5. It also says $underFDL is required (even thought that is what the doc has. Finally, it says "Prefer '' over '&kappname;'" (should it have what is preferred?) even though I can find no more instances of &kappname.
(I wonder if the problem here is because this is in extragear?)