I've been building something similar which should also be used as a starting point for web development.

It includes a setup for simple plain CSS that can scale. It's split into a reset.css, var.css, global.css, util.css and markdown.css.

Using that approach for this Astro Starter Template: https://starter.bryanhogan.com/

I describe the setup in more detail here: https://webdev.bryanhogan.com/css/css-setup/