it's not really clear what sets it apart from vanilla CSS at the moment

is NueCSS an actual concrete "thing" (e.g. will there be docs? a library?) or it's just a back-to-basics method that fits in with the other parts of Nue framework and the future proposed Design System?

The most important thing that sets apart Nue from other frameworks is indeed vanilla CSS. Think Nue as the ideal framework for using vanilla CSS with hot-reloading in place for all asset types. Modern development is shifting towards CSS-in-JS or Tailwind, which take you further from the design system thinking.

I see two stances towards Nue:

1) You are an advocate of React, CSS-in-JS, and think separation of concerns is not important. Then I’ve experienced that it’s impossible to get Nue, and I don’t even attempt to explain it in more detail. The situation is the worst (in terms of explaining Nue) if you think Tailwind is the best thing that has happened to CSS.

2) If you feel the separation of concerns is one of the most important things in software development and actually like CSS (instead of hate it), then Nue is likely something you might want. Or if there is a better one, I'm very eager to hear about it.

Here is the ultimate goal and the “big edge” you might be looking for:

https://nuejs.org/blog/perfect-web-framework/

so... NueCSS isn't a "thing" itself so much, it's just saying that the concept of Nue framework is that you will write vanilla CSS based off a Design System?

Or more concretely: "A new, minimalistic way to write vanilla CSS"

what does this part mean?

May i recommend - https://github.com/gnat/css-scope-inline I am using it in a project and it has been working quite well. And i think it would work along with the Nue design system as well.