Tailwind CSS v4 vs v3: Major Changes & Migration Guide

The 2026 Paradigm Shift: CSS-First Engine

For years, Tailwind relied on a heavy JavaScript-side processor to generate CSS. Tailwind v4 flips the script by using Lightning CSS to perform the heavy lifting directly at the CSS level.

### Key Differences in Configuration

- v3: You manage colors, spacing, and themes in a tailwind.config.js file.

- v4: You define your theme directly in your CSS file using the @theme block. This makes your theme part of the native CSS variable registry.

### Why Build Times are 10x Faster

Because v4 is built on Rust (via Lightning CSS) and avoids the JS-to-CSS translation layer, the JIT (Just-In-Time) engine is nearly instantaneous. This is particularly noticeable in large Next.js 15 projects where HMR (Hot Module Replacement) latency used to be a bottleneck.

### Should You Upgrade?

If you are using basic Tailwind features, the upgrade is seamless. However, if your project relies on complex JavaScript-driven plugins in your config, you may need a bridge or wait for those plugins to release v4-native versions.