Almost every Tailwind 5,202 class follows one grammar: optional variants (conditions such as hover: or md:), a utility naming a CSS property, a value from your theme or in brackets, and an optional modifier after a slash. The class below compiles to one rule, guarded by the dark, 48rem and hover conditions, that sets background-color: color-mix(in oklab, var(--color-sky-500) 50%, transparent).
The rest lives in CSS: @import "tailwindcss" loads the default theme, Preflight and utilities into cascade layers (@layer and @scope), and directives such as @theme and @source decide which classes exist. Demos use the Play CDN build (Installation Options, version 4.3.3), with directives in <style type="text/tailwindcss">.