postcss-plugins VS postcss-define-function

the author of the most popular PostCSS plugin himself recommended the postcss-preset-env over his own creation which is cssnex, and

The example in A) won't work as the plugin can't convert the functions with variables in it. https://github.com/csstools/postcss-plugins/tree/main/plugin...

You can use this today if you're using PostCSS in your build process (which is the case if you use autoprefixer)! https://preset-env.cssdb.org/

Switching from a ready-made tool like Sass or a recommendation package like cssnext (deprecated since 2019) or PostCSS Preset Env (archived in 2022), to the modular PostCSS Preset Env plugin set we can choose a helpful and convenient set of future CSS features beyond the current stable client CSS.

To implement :has support, the css-has-pseudo polyfill can be used. This polyfill includes both a PostCSS plugin and a JS function to be used in the HTML page.

Yes, it's behind a flag and only in future versions, but it is a huge step forward. Those versions will soon be available, and we can try it locally (unfortunately, not on production without a polyfill or a plugin).

w/ postcss-preset-env(v7.8.3): convert modern CSS into something most browsers can understand, determining the polyfills you need based on your targeted browsers or runtime environments. It takes the support data that comes from MDN and Can I Use and determine from a browserlist whether those transformations are needed. It also packs Autoprefixer within and shares the list with it, so prefixes are only applied when you're going to need them given your browser support list.

There's a @ custom-media proposal (and PostCSS plugin) that can make it a lot easier to use media queries across components. Define your MQs in your global CSS, use everywhere. If you're worried about repeating MQs in your CSS from a bloat perspective, there are PostCSS plugins for that as well.

The term you are looking for is "nesting". CSS currently does not support it. But there is a draft being worked on. No browser currently supports it, though. Most CSS Pre- or Postprocessors like Sass, Less, Stylus, PostCSS support nesting.

User defined functions might become part of CSS one day, but currently there isn't even a working draft. So I would contradict my claim at least in this aspect, when I told you to use the PostCSS define-function plugin which mimics a subset of SCSS's function syntax.