"Every time I think I'm out... they pull me back in!"
I'm so glad you mentioned that Pagina epubchecker -- I had indeed heard about it before, but apparently forgot about it. Just installed it this morning and found a whole bunch of errors that for some reason the Sigil validator hadn't picked up on! And re your online validator, I had come across that before (or one like it), but my book is 14 megs in size, and yours can only handle up to 10 megs -- it's so much nicer if I can just validate off my hard drive, though, of course. I guess Pagina should do the trick, in that regard.
Okay, you've inspired me again... I think... maybe... I'm not sure.
I've been spending so much time recently trying to resolve "problems," though, that I've been feeling terribly neglectful about just simply getting the textual parts of my book finished.
In other words, I do think that I
will take another stab at trying to convert it to epub3 -- probably by "cheating," taking your valid book and copying/pasting my stuff into yours (and making whatever changes along the way, as necessary) -- but first I think I just need to get my epub2 version finished up, and at least have all the writing, editing, formatting and arranging over and done with.
So I'll get back to you! Might take me a while -- anything from days to weeks (I'm also waiting for one final article/chapter from another person) -- but I will get back to this, I'm sure (and back here again when I run into the inevitable problems).