Programming, I'm guessing. Page layout was its own weird world even before HTML came along, and the explosion of devices and use cases has made the domain much weirder. Put 25 years of CSS evolution on top of that and it's very much its own beast. As somebody who's done mostly back-end work lately, whenever I approach CSS it's one WTF after another.

I keep hearing that it has settled down, though, so I am looking forward to learning it properly next I need to do something interface-y.