Tailwind v4 is a significant rewrite. The biggest change: no more tailwind.config.js.
CSS-first configuration
Everything that used to live in tailwind.config.js now lives in your CSS file:
@import "tailwindcss";
@theme {
--color-primary: #4f46e5;
--font-sans: system-ui, sans-serif;
}
\```
Cleaner. One less file to maintain.
## The dark mode change
In v3, you'd add `darkMode: 'class'` to the config. In v4, you declare it in CSS:
```css
@variant dark (&:where(.dark, .dark *));
\```
Same result, different location. This tripped me up when first migrating — the dark: classes compiled fine but never activated until I found this line.
## Performance
Build times are significantly faster in v4. The new Oxide engine rewrites the CSS scanning in Rust. On this project, builds went from ~2s to ~700ms.
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