Tailwind CSS v4: CSS-First Configuration And The New Engine

**Tailwind CSS v4** (released in 2025, adopted widely in 2026) reworks installation around a single CSS import (`@import "tailwindcss"`) and pushes configuration toward CSS-native `@theme` blocks instead of a mandatory `tailwind.config.js` for many projects. The Oxide engine (Rust) speeds incremental builds in large repos.

Utility classes still compile from your source scanning, but defaults and design tokens integrate with modern CSS features (custom properties, `@layer`). Plugins from the v3 era may need updates; check each plugin's compatibility notes.

Migration path from v3: follow `tailwindcss.com/docs/upgrade-guide`, run the official upgrade tool where provided, and diff generated CSS in CI. Pin versions during migration sprints.

Tailwind is a styling system, not a component library. Pair it with accessible primitives (Radix, React Aria) when building interactive widgets.