Maybe I am an outlier... but I really don't like nested style documents like those founds with SASS, LESS, PostCSS. When the number of nested selectors becomes too great it can become very difficult to reason about. I would never use this without some kind of lint rule enforcing a maximum depth of selector nesting. I agree with the other commenters ITT taking the position flat CSS looks cleaner than nested documents.
> When the number of nested selectors becomes too great it can become very difficult to reason about. This is a sign you need to decompose whatever design you are working on into smaller components. If you’re trying to write styles for a whole page at once, you’ll see this problem, but if you are working on small components, it’s very rare.
I haven't followed modern CSS development much. Does it have a module system now?