Not sure, but the EPUBreader plugin or Firefox
may or may not support ePub3. I wrote to Michael, the dev, to find out.
I also found an ePub 3 validator add-on for FF
that you may want to try. Not sure if it's any good.
Hmm, on second thought, I just got my Ativ 700 today, not sure if FF works in metro UI, as I haven't had much time to test yet.