Register Sources Only Where Detection Cannot Reach
MUST: Configure Tailwind content paths to include all files using utility classes
Tailwind v4 auto-detects sources — add @source only for files outside that sweep
Bad
Good
Why it matters
There is no `content` array in Tailwind v4. It was removed. Tailwind now walks the project itself, starting from where the CSS is imported, skipping anything in .gitignore and every binary/asset extension — so in a normal app the correct configuration is none at all, and pasting a v3 `content: [...]` block does nothing but mislead the next reader. Reach for `@source "…"` in exactly one situation: a file that holds class names but that the sweep cannot see — a component library inside node_modules (which is gitignored), or a template directory outside the project root.
`@source "../node_modules/@acme/ui/dist"`. Two escape hatches complete the picture: `@source not "…"` excludes a path the sweep found but should not scan (a huge generated fixture directory), and `@import "tailwindcss" source(none)` disables auto-detection entirely so you can list every source explicitly — worth it only when you genuinely need that control.