cssnano is a production-grade CSS minifier built as a modular PostCSS plugin system. It takes CSS input and applies a configurable chain of optimizations—color reduction, value normalization, selector minification, longhand merging, rule merging, whitespace stripping, duplicate removal, and more—to produce the smallest possible CSS output while preserving semantics and browser compatibility. The architecture is a monorepo of over 30 PostCSS plugins, orchestrated by three built-in presets (lite, default, advanced) that balance aggressiveness against safety. The codebase is exceptionally well-engineered: every optimization is driven by data extracted from CSS specifications (via @webref/css) and browser compatibility data (via @mdn/browser-compat-data), ensuring transformations only apply when safe for target browsers. Extensive testing includes unit tests, integration tests against major CSS frameworks, fuzz testing with invariants (idempotency, cascade preservation), mutation testing, and statistical performance benchmarking. The project demonstrates exceptional craftsmanship in its handling of CSS value grammars, cascade-aware merging algorithms, and browser-specific fallbacks.