Tailwind CSS v4: What Actually Changed and How I Migrated Two Projects

Headline: Tailwind v4 is the most significant rewrite since the framework launched — CSS-first config, Lightning CSS under the hood, container queries built-in, and no more tailwind.config.js. I migrated two production projects and here's what actually broke and what the upgrade tool misses.

Tailwind CSS v4 arrived with a steeper upgrade curve than most version bumps in the JS ecosystem. The configuration story changed completely. The build engine changed. Several features that previously required plugins are now built-in.

The headline change: no more tailwind.config.js

In v3, configuration lived in a JavaScript file — theme extensions, plugins, content paths. In v4, it moves into your CSS:

@import "tailwindcss";