yeah, "supposed to" is doing a quite a bit of heavy lifting. it has gotten soooo much better than in the past, but yet things still do slightly weird things depending on browser. the closer to the designer you are, the more you notice. by notice, i mean have it pointed out to you.
It's not just about the engine itself, it's about things like supporting extensions. Safari's content blocker API is less powerful than Firefox's on desktop/Android, for example. I don't know how much of that is a technical engine limitation versus an Apple policy decision.
