[akerl_]

The Recurse Center, and Julia Evans, have correctly identified that it's a net negative social practice for people to on-the-fly decide that somebody needs to be mocked for not knowing something, regardless of how much you think they ought to know it.

Even your "calm" version probably doesn't need to exist. If there's something they want to do and they're asking you about how to do it, by all means, it may be relevant to tell them that learning a new thing would potentially help them.

Otherwise maybe worry less about what other people should or shouldn't know.

[isoprophlex]

Indeed. There's nothing lost by rephrasing that to "Aha, good to know that you have not heard of CSS before! That means I will explain you some basics first, otherwise x and y will be incomprehensible to you."

[Aurornis]

Honestly I wouldn't even say that much. The point of the rule is that there's no need to call it out or even discuss the fact that you know it but they don't.

Just go straight to explaining it helpfully. Don't make the knowledge gap itself a point of discussion at all.

[bazoom42]

Still comes off as rather condescending, honestly.

Why not just explain the thing that needs explaining.