Yes, I think that an epub3 builder can implement some of the improvements of epub3 without a proper HTML5 builder.
So the pull request is fine, although I have a few comments, that are marked inline.
I would like to keep two different buiders, the minimal epub2 builder that produces small epubs for resource starved
Even if an epub2+3 compatible format is possible, it looks a little tricky, so for the moment, I would
stick with the two builders.