Migrate data theme keys (#18816)

This PR is similar to and a follow up of #18815, but this time to
migrate the `data` theme keys.

Let's imagine you have the following Tailwind CSS v3 configuration:
```ts
export default {
  content: ['./src/**/*.html'],
  theme: {
    extend: {
      data: {
        // Automatically handled by bare values
        foo: 'foo',
    //  ^^^   ^^^       ← same names

        // Not automatically handled by bare values
        bar: 'baz',
    //  ^^^   ^^^       ← different names

        // Completely custom
        checked: 'ui~="checked"',
      },
    },
  },
}
```

Then we would generate the following Tailwind CSS v4 CSS:

```css
@custom-variant data-bar (&[data-baz]);
@custom-variant data-checked (&[data-ui~="checked"]);
```

Notice how we didn't generate a custom variant for `data-foo` because
those are automatically handled by bare values.

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Discard matched variants with non-string values ([#18799](https://github.com/tailwindlabs/tailwindcss/pull/18799))
 - Show suggestions for known `matchVariant` values ([#18798](https://github.com/tailwindlabs/tailwindcss/pull/18798))
 - Migrate `aria` theme keys to `@custom-variant` ([#18815](https://github.com/tailwindlabs/tailwindcss/pull/18815))
+- Migrate `data` theme keys to `@custom-variant` ([#18816](https://github.com/tailwindlabs/tailwindcss/pull/18816))
 
 ## [4.1.12] - 2025-08-13
 

integrations/upgrade/js-config.test.ts

@@ -1048,6 +1048,80 @@ test(
   },
 )
 
+test(
+  'migrate data theme keys to custom variants',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "tailwindcss": "^3",
+            "@tailwindcss/upgrade": "workspace:^"
+          }
+        }
+      `,
+      'tailwind.config.ts': ts`
+        export default {
+          content: {
+            relative: true,
+            files: ['./src/**/*.html'],
+          },
+          theme: {
+            extend: {
+              data: {
+                // Automatically handled by bare values
+                foo: 'foo',
+
+                // Not automatically handled by bare values because names differ
+                bar: 'baz',
+
+                // Custom
+                checked: 'ui~="checked"',
+              },
+            },
+          },
+        }
+      `,
+      'src/input.css': css`
+        @tailwind base;
+        @tailwind components;
+        @tailwind utilities;
+      `,
+    },
+  },
+  async ({ exec, fs, expect }) => {
+    await exec('npx @tailwindcss/upgrade')
+
+    expect(await fs.dumpFiles('src/*.css')).toMatchInlineSnapshot(`
+      "
+      --- src/input.css ---
+      @import 'tailwindcss';
+
+      @custom-variant data-bar (&[data-baz]);
+      @custom-variant data-checked (&[data-ui~="checked"]);
+
+      /*
+        The default border color has changed to \`currentcolor\` in Tailwind CSS v4,
+        so we've added these compatibility styles to make sure everything still
+        looks the same as it did with Tailwind CSS v3.
+
+        If we ever want to remove these styles, we need to add an explicit border
+        color utility to any element that depends on these defaults.
+      */
+      @layer base {
+        *,
+        ::after,
+        ::before,
+        ::backdrop,
+        ::file-selector-button {
+          border-color: var(--color-gray-200, currentcolor);
+        }
+      }
+      "
+    `)
+  },
+)
+
 describe('border compatibility', () => {
   test(
     'migrate border compatibility',

packages/@tailwindcss-upgrade/src/codemods/config/migrate-js-config.ts

@@ -152,6 +152,18 @@ async function migrateTheme(
       }
       delete resolvedConfig.theme.aria
     }
+
+    if ('data' in resolvedConfig.theme) {
+      for (let [key, value] of Object.entries(resolvedConfig.theme.data ?? {})) {
+        // Will be handled by bare values if the names match.
+        // E.g.: `data-foo:flex` should produce `[data-foo]`
+        if (key === value) continue
+
+        // Create custom variant
+        variants.set(`data-${key}`, `&[data-${value}]`)
+      }
+      delete resolvedConfig.theme.data
+    }
   }
 
   // Convert theme values to CSS custom properties
@@ -223,7 +235,13 @@ async function migrateTheme(
 
   if (variants.size > 0) {
     css += '\n@tw-bucket custom-variant {\n'
+
+    let previousRoot = ''
     for (let [name, selector] of variants) {
+      let root = name.split('-')[0]
+      if (previousRoot !== root) css += '\n'
+      previousRoot = root
+
       css += `@custom-variant ${name} (${selector});\n`
     }
     css += '}\n'
@@ -389,7 +407,7 @@ const ALLOWED_THEME_KEYS = [
   // Used by @tailwindcss/container-queries
   'containers',
 ]
-const BLOCKED_THEME_KEYS = ['supports', 'data']
+const BLOCKED_THEME_KEYS = ['supports']
 function onlyAllowedThemeValues(theme: ThemeConfig): boolean {
   for (let key of Object.keys(theme)) {
     if (!ALLOWED_THEME_KEYS.includes(key)) {