Following https://dev.37signals.com/modern-css-patterns-and-techniques-in-campfire/

More thoughts about utility classes (only utilities and not a core use anymore), :has(),

What fascinated me most was watching the architecture evolve across releases.

Campfire: OKLCH colors, Custom properties for everything, character-based spacing, flat file oranization, View Transitions API
Writebook (2nd): COntainer queries for component-level responsiveness, @starting-style for entrance animations
Fizzy (3rd release): CSS Layers, color-mix() and complex :has() to replace JS.

37Signals share their product codes in Open Source. That's awesome because we can learn from it:

/* Fizzy's layer architecture */
@layer reset, base, components, modules, utilities;

@layer components {
  .btn { /* Always lower specificity than utilities */ }
}

@layer utilities {
  .hide { /* Always wins over components */ }
}

A CSS only spinner under 30 lines of CSS code. "Pure creativity".
A better <mark> that draws a hand-drawn circle around matched terms.
They also created dialog animations in CSS only.