fix(@tailwindcss/vite): include @variant in feature detection (#19966)

Fixes #19964

CSS files imported via JS that only use `@variant` (no `@apply`,
`theme()`, or utility classes) are silently skipped by the Vite plugin.
The `@variant` directive gets passed through raw to the browser, which
drops it as an unknown at-rule.

**Root cause** — `Features.Variants` is missing from the [feature
detection
bitmask](https://github.com/tailwindlabs/tailwindcss/blob/v4.2.4/packages/%40tailwindcss-vite/src/index.ts#L526):

```ts
// before
Features.AtApply | Features.JsPluginCompat | Features.ThemeFunction | Features.Utilities

// after
Features.AtApply | Features.JsPluginCompat | Features.ThemeFunction | Features.Utilities | Features.Variants
```

**Reproduction** — https://github.com/remorses/tailwind-variant-bug

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Ensure `@plugin` resolves package JavaScript entries instead of browser CSS entries when using `@tailwindcss/vite` ([#19949](https://github.com/tailwindlabs/tailwindcss/pull/19949))
 - Fix relative `@import` and `@plugin` paths resolving from the wrong directory when using `@tailwindcss/vite` ([#19965](https://github.com/tailwindlabs/tailwindcss/pull/19965))
+- Ensure CSS files containing `@variant` are processed by `@tailwindcss/vite` ([#19966](https://github.com/tailwindlabs/tailwindcss/pull/19966))
 
 ## [4.2.4] - 2026-04-21
 

packages/@tailwindcss-vite/src/index.ts

@@ -537,7 +537,11 @@ class Root {
     if (
       !(
         this.compiler.features &
-        (Features.AtApply | Features.JsPluginCompat | Features.ThemeFunction | Features.Utilities)
+        (Features.AtApply |
+          Features.JsPluginCompat |
+          Features.ThemeFunction |
+          Features.Utilities |
+          Features.Variants)
       )
     ) {
       return false