I think it’s great that the CSS Working Group does these. It’s like planting a flag in the ground saying this is what CSS looks like at this specific point in time. They do specifically say it’s not for us CSS authors though…
This document collects together into one definition all the specs that together form the current state of Cascading Style Sheets (CSS) as of 2020. The primary audience is CSS implementers, not CSS authors, as this definition includes modules by specification stability, not Web browser adoption rate.
Remember “CSS3”? That was the closest thing we had to a “snapshot” that was designed for CSS authors (and learners). Because CSS3 was so wildly successful, we saw a short round of enthusiasm for CSS4, me included. There is zero marketing panache on that snapshot page, which is exactly what CSS4 would need to succeed. Remember, HTML5 and friends (including CSS3) even had fancy logos!
If someone were to say to me “Chris, when CSS3 came around, I boned up on all that, but I haven’t kept up with CSS since, what should I learn?” I’d say “That’s a damn fine question, developer that has a normal healthy relationship with technology.” But honestly, I might struggle to answer cohesively.
I’d say: Uhm, CSS grid for sure. Custom properties. Clipping and Offset paths I suppose.
prefers-reduced-motion. I dunno. There are probably like 100 things, but there is no great single reference point to see them all together.
I’ll work on putting a list together. I don’t think I’ll have the gumption to call it CSS4, but at least I’ll be able to answer that question. Feel free to suggest ideas in the comments.