Ant Design CSS-in-JS

@ant-design/cssinjs is the CSS-in-JS runtime that Ant Design itself uses to turn design tokens into scoped, cached component styles. It’s described by its own maintainers as a purpose-built subset of Emotion with a design-token layer wrapped around it: components register style functions via useStyleRegister, results are hashed and cached per token combination, and a StyleProvider controls where and how styles are injected (including into Shadow DOM or a custom container).

Beyond basic style injection, the library ships a theme system (createTheme, Theme) for deriving computed design tokens, CSS-variable support so tokens can update without re-computing styles, a set of transformers (legacy logical properties, px-to-rem, vendor auto-prefixing) for output normalization, and lint-style checks that catch common CSS-in-JS mistakes (NaN values, overly broad selectors) during development. extractStyle supports server-side rendering by pulling all cached styles out as a string for critical-CSS injection.

What You Get

• useStyleRegister/useCSSVarRegister hooks that hash design tokens into cached, deduplicated CSS injected once per unique combination
• A StyleProvider component controlling injection container, CSS-variable mode, hash priority, and SSR inline-style fallback
• A theme system (createTheme, Theme, calculators) for deriving and caching computed design tokens from a base algorithm
• extractStyle for pulling all cached styles out as a string during server-side rendering
• Built-in transformers (legacy logical properties, px-to-rem, auto-prefixing) and dev-mode linters (NaN values, overly broad selectors, quote-less content) for output correctness

Common Use Cases

• Building a design-system component library that needs token-driven, deduplicated runtime CSS like Ant Design’s own components
• Adding SSR-safe critical CSS extraction to a CSS-in-JS-based React app to avoid flash-of-unstyled-content
• Injecting component styles into a Shadow DOM boundary (web components) via the container option on StyleProvider
• Normalizing generated CSS for older browsers using the px2rem or legacy logical-properties transformers

Under The Hood

Architecture - The runtime is organized around a small set of composable primitives: StyleContext.tsx defines StyleProvider/createCache (the injection boundary and cache entity), hooks/useStyleRegister.tsx and hooks/useCacheToken.tsx do the actual token-hashing and <style> injection/dedup, theme/ implements the token-derivation and calculator system (Theme.ts, ThemeCache.ts, calc/), and extractStyle.ts walks the cache to serialize styles for SSR. transformers/ and linters/ are pluggable post-processing/validation passes applied to the generated CSSObject before it’s stringified via stylis. Tech Stack - TypeScript throughout, built with father (Umi’s library bundler) to CJS/ESM, styling primitives from stylis (CSS preprocessing), @emotion/hash and @emotion/unitless (borrowed directly from Emotion’s internals per the README), and @rc-component/util for shared React-ecosystem helpers; React/React DOM are peer dependencies (>=16), and tests run on Vitest with @testing-library/react and jsdom. Code Quality - Test coverage is broad relative to the library’s surface: 14 spec files under tests/ cover caching (cache.spec.tsx), theming (theme.spec.tsx, calc.spec.tsx), SSR extraction and ordering (server.spec.tsx, server-order.spec.tsx), CSS variables, transformers, keyframes, and linters individually. Code is organized into small single-responsibility modules (one file per hook, one file per transformer/linter) rather than large monoliths, with typed public interfaces (CSSObject, CSSInterpolation, Transformer, Linter) exported directly from src/index.ts. API Design - The public API surface is deliberately narrow and hook-first (useStyleRegister, useCSSVarRegister, useCacheToken) paired with a single StyleProvider for global configuration, which keeps integration low-boilerplate for consumers already using token-based theming, though the caching-for-performance design does impose “strong constraints” the README itself flags as something to study via the example demos before extending.