what's new and how to upgrade

Tailwind CSS v4.0, released January 22, 2025, is a major overhaul of the utility-first CSS framework. Key changes include replacing tailwind.config.js with a CSS-native @theme directive, integrating Lightning CSS (a Rust-based engine) for up to 5× faster full builds and microsecond incremental builds, and simplifying setup to a single @import statement. New features include container queries, 3D transform utilities, expanded gradient APIs, and modern CSS capabilities like cascade layers, @property, and color-mix(). The release is a breaking change from v3, but an official upgrade codemod (@tailwindcss/upgrade) is available to assist migration.