Thanks, I couldn't get it through the link since it's behind a "text fragment" link which only works on Chrome[1], a bit ironic if you ask me when we are discussing a standard :)
[1] https://developer.mozilla.org/en-US/docs/Web/HTML/Element/de... [2] https://caniuse.com/details
https://www.w3.org/TR/css-nesting-1/#:~:text=Why%20can%E2%80...
https://www.w3.org/TR/css-nesting-1/#nesting
The W3C document template even kindly provides a quick way to copy those links to every header using the section mark (§).