Tailwind CSS v4 migration: switch without downtime
An executable path for moving an existing project to Tailwind CSS v4, with a rollback plan.
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Quick answer
What is the biggest config difference between Tailwind v4 and v3?
v4 drops tailwind.config.js and declares themes entirely in CSS via @theme. Upgrading requires deleting the old config and installing @tailwindcss/vite — otherwise the build fails with a missing module.
Machine-readable page data: /data/tailwindcss-migration.json
In-depth analysis
Tailwind CSS v4 replaces the bulky JavaScript config file with a CSS-first @theme directive, improving both build speed and control. Migration runs in parallel with validation and gradual cutover — keep a switch that rolls back in seconds so business never breaks.
/* src/styles/global.css — Tailwind v4 CSS-first config, no tailwind.config.js needed */
@import 'tailwindcss';
@theme {
--color-brand: oklch(0.62 0.19 265);
--font-sans: 'Inter', system-ui, sans-serif;
}Tailwind CSS v4 Migration · Key takeaways
- Full branch build passes
- All 8 Preview drills green
- Production and Preview KV isolated
- Rollback switch ready
FAQ & pitfalls
How do I long-cache Tailwind v4 build output at the edge?
The compiled CSS uses content-hashed filenames with immutable long cache. HTML references the new hash automatically, so no manual cache busting is required.
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What readers are asking
Questions that keep coming back in reader feedback, already answered.
- 01
Why is my custom Tailwind v4 brand colour not applied?
Colours must be declared in @theme (for example --color-brand). A missing @tailwindcss/vite plugin, or a leftover v3 config, makes bg-brand silently render nothing.
See the full guide → - 02
Why is my Astro component's interactivity not working?
Components ship as zero-JS HTML by default, so an interactive one needs a client:* directive (client:load, client:visible, …). Without it the script is never bundled into the page.
See the full guide → - 03
Why are _headers / _redirects not taking effect?
Both files must sit at the root of the output directory (dist/); anything under src/ is never uploaded. A purely static project should not add _worker.js either — that gives up the zero-cold-start advantage.
See the full guide →