When I started dabbling in web development, writing HTML and CSS was already its own job, and professional JavaScript developers basically did not exist. This was before TypeScript, before Node, before Ajax, before React or even jQuery. If anything has exploded in complexity in the intervening years, it's the JavaScript part of the equation.
I agree that it's increasingly silly to ask someone to be an expert in all of frontend. But the primary driver of that is not all the new CSS features we're getting.
Agreed. Having a "HTML + CSS" engineer on the team was largely due to the number of hacks needed to make css work -- purposely adding invalid html that would only be parsed by specific browsers, ie5 vs ie6 vs netscape being wildly different (opera mobile was out of this world different), using sprites everywhere because additional images would have a noticeable lag time (especially with javascript hover), clearfix divs to overcome float issues. To be clear, I'm not saying "things were harder back then" or "css is simple now", but things with CSS were so wild and the tooling was so bad, that it what a unique skill of it's own that is less needed now, and the shift has been for people to focus on going deeper with js.