Tailwind CSS v4 — What Actually Changed (And What It Means for Your Next.js Project)

Tailwind v3 had a good run. Drop in tailwind.config.js , point the content array at your files, wire up PostCSS — done. Then v4 shipped in early 2025 and it's not an incremental update. It's a rethink. I've been running v4 in a real Next.js project ( QuickPU result portal ) and this is what actually changed in practice. The Biggest Shift — CSS-First Configuration In v3, your design system lived in JavaScript. In v4, it lives in CSS. v3 setup — two config files, PostCSS required: // tailwind.conf