Unfortunately I've just found a book that the test plugin has slightly mangled, producing invalid HTML in two files.
I'm attaching a zip containing the original files, which look like they are nearly XHTML except I thought empty XHTML elements had a start tag ending with " />". These just end "/>", i.e. without the space. But if you're using a real XML parser that shouldn't matter.
In both cases the plugin leaves an open <div> element at the end of the text. Sigil complains, and the FlightCrew validator complains.
On the plus side, at least the bogus GBS anchors are gone
