Fascinating. I didn't know about the @supports at-rule... I can see how that would be a real game changer.
EDIT: I've been rolling this around in my head for the last thirty minutes: Does your example above mean that—depending on the epub3 code you're trying to accommodate—the epub file might require having functionally the whole book duplicated inside so that it could toggle your at-rule? Or could you stack the two classes, and have the toggling still work as in:
Code:
<div class="epub3Code epub2Code">
All the code for epub3 AND epub2 here...
</div>