Yeah, that's what I decided to do, like five minutes after I posted my previous answer. Because I simply cannot stand not knowing in cases such as this, especially when I know it's a mess of my own making, so of course I'd keep trying and trying. It took a while, but I think I finally found the culprit:
Apparently putting margin in there was a big, big mistake.
With a mistake like that, naturally not even epubcheck or css validators will be useful.
I don't even remember why I thought using that css line would be a good idea, when what I want is more easily accomplished with this:
tl;dr: So yeah, case closed! Finally!
Thank you for all your efforts
