Author here. Like a lot of people on HN, Dave Shea's CSS Zen Garden is how I learned CSS in the first place. Building stylesheets for that one HTML file taught me to think about CSS as a system, and it stuck. Nearly two decades later I got to build the design system for the new Firefox.com together with a team from Lincoln Loop, on a contract engagement with Mozilla using pure, modern CSS with no preprocessors (almost). In the end, we did run a few things through preprocessors for performance reasons, but the site's CSS is fully authored in pure CSS syntax.

Happy to answer questions about the layered CSS, the legacy browser fallback, how the tokens got exported from design files, or anything else you're curious about.