It’s not apparent from the document posted, but this is actually almost six years old [0] and already implemented as a PostCSS plugin for just as long [1]. It was adopted by the CSS Working Group a couple of years ago [2]. So this is very well-established and you’ve been able to use this syntax for many years. But it’s good to see it moving forward and hopefully browsers will implement it soon now.
This is good historical context but missing one bit and another deserves clarification.
The nesting semantics has been a part of SASS/SCSS for the better part of a decade before the proposal, which is now pretty much standard in CSS pre-/post-processing.
It’s been available to use all that time, but AFAIK still requires a build tool. It’s only now becoming a tentative possibility in userland.
When I first saw this link I thought “Hang on a sec., this says ‘First Public Working Draft’, but hasn’t this spec. been around for ages?”
I figured people might make the mistake that this was something new that the W3C were only just getting around to rather than something that has been cooking for a long time.
Didn’t mean to imply that the spec. itself sprang up out of nowhere – it was definitely based on the Sass work that came before it, and you have been able to use this syntax with Sass for a very long time!
[0] https://tabatkins.github.io/specs/css-nesting/
[1] https://github.com/csstools/postcss-nesting
[2] https://github.com/w3c/csswg-drafts/pull/2878