Thanks Tommy, as complex as this mess is, supposedly it's completely valid (or soon shall be). Unfortunately, this is what happens when you let W3C people get involved with something. DAISY predated OEBF, and was more or less its own thing (and dramatically simpler) in DAISY 2. Then someone decided that DAISY and OEBF should merge, since they're both books right?, and suddenly OEBF is supposed to be more than just an extended web browser, it's now a full multimedia-enabled extended web browser!
Consequently, many DAISY 2 playback softwares, even those capable of text-to-speech were not updated for DAISY 3, because a properly compliant DAISY 3 reader is more ambitious a project than writing a web browser, unless you can just shove a whole web browser core into your code and start extending it, of course. Those that exist aren't properly compliant and just ignore outright any tag they don't understand.
But that's a whole 'nother rant. I remain convinced that e-books should be an HTML subset or a DTD similar to an HTML subset with obvious book tags added, not an HTML superset.
Nobody cares what I think though.
Anyway, I'm looking forward to some future opf2mobi that can handle this madness.