Support loading config files via @config (#14239)

In Tailwind v4 the CSS file is the main entry point to your project and
is generally configured via `@theme`. However, given that all v3
projects were configured via a `tailwind.config.js` file we definitely
need to support those. This PR adds support for loading existing
Tailwind config files by adding an `@config` directive to the CSS —
similar to how v3 supported multiple config files except that this is
now _required_ to use a config file.

You can load a config file like so:

```
@import "tailwindcss";
@config "./path/to/tailwind.config.js";
```

A few notes:
- Both CommonJS and ESM config files are supported (loaded directly via
`import()` in Node)
- This is not yet supported in Intellisense or Prettier — should
hopefully land next week
- TypeScript is **not yet** supported in the config file — this will be
handled in a future PR.

---------

Co-authored-by: Philipp Spiess <hello@philippspiess.com>
Co-authored-by: Adam Wathan <adam.wathan@gmail.com>
Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - Add new standalone builds of Tailwind CSS v4 ([#14270](https://github.com/tailwindlabs/tailwindcss/pull/14270))
+- Support JavaScript configuration files using `@config` ([#14239](https://github.com/tailwindlabs/tailwindcss/pull/14239))
 
 ### Fixed
 

integrations/cli/config.test.ts

@@ -0,0 +1,191 @@
+import { candidate, css, html, js, json, test } from '../utils'
+
+test(
+  'Config files (CJS)',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/cli": "workspace:^"
+          }
+        }
+      `,
+      'index.html': html`
+        <div class="text-primary"></div>
+      `,
+      'tailwind.config.js': js`
+        module.exports = {
+          theme: {
+            extend: {
+              colors: {
+                primary: 'blue',
+              },
+            },
+          },
+        }
+      `,
+      'src/index.css': css`
+        @import 'tailwindcss';
+        @config '../tailwind.config.js';
+      `,
+    },
+  },
+  async ({ fs, exec }) => {
+    await exec('pnpm tailwindcss --input src/index.css --output dist/out.css')
+
+    await fs.expectFileToContain('dist/out.css', [
+      //
+      candidate`text-primary`,
+    ])
+  },
+)
+
+test(
+  'Config files (ESM)',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/cli": "workspace:^"
+          }
+        }
+      `,
+      'index.html': html`
+        <div class="text-primary"></div>
+      `,
+      'tailwind.config.mjs': js`
+        export default {
+          theme: {
+            extend: {
+              colors: {
+                primary: 'blue',
+              },
+            },
+          },
+        }
+      `,
+      'src/index.css': css`
+        @import 'tailwindcss';
+        @config '../tailwind.config.mjs';
+      `,
+    },
+  },
+  async ({ fs, exec }) => {
+    await exec('pnpm tailwindcss --input src/index.css --output dist/out.css')
+
+    await fs.expectFileToContain('dist/out.css', [
+      //
+      candidate`text-primary`,
+    ])
+  },
+)
+
+test(
+  'Config files (CJS, watch mode)',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/cli": "workspace:^"
+          }
+        }
+      `,
+      'index.html': html`
+        <div class="text-primary"></div>
+      `,
+      'tailwind.config.js': js`
+        const myColor = require('./my-color')
+        module.exports = {
+          theme: {
+            extend: {
+              colors: {
+                primary: myColor,
+              },
+            },
+          },
+        }
+      `,
+      'my-color.js': js`module.exports = 'blue'`,
+      'src/index.css': css`
+        @import 'tailwindcss';
+        @config '../tailwind.config.js';
+      `,
+    },
+  },
+  async ({ fs, spawn }) => {
+    await spawn('pnpm tailwindcss --input src/index.css --output dist/out.css --watch')
+
+    await fs.expectFileToContain('dist/out.css', [
+      //
+      candidate`text-primary`,
+      'color: blue',
+    ])
+
+    await fs.write('my-color.js', js`module.exports = 'red'`)
+
+    await fs.expectFileToContain('dist/out.css', [
+      //
+      candidate`text-primary`,
+      'color: red',
+    ])
+  },
+)
+
+test(
+  'Config files (MJS, watch mode)',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/cli": "workspace:^"
+          }
+        }
+      `,
+      'index.html': html`
+        <div class="text-primary"></div>
+      `,
+      'tailwind.config.mjs': js`
+        import myColor from './my-color.mjs'
+        export default {
+          theme: {
+            extend: {
+              colors: {
+                primary: myColor,
+              },
+            },
+          },
+        }
+      `,
+      'my-color.mjs': js`export default 'blue'`,
+      'src/index.css': css`
+        @import 'tailwindcss';
+        @config '../tailwind.config.mjs';
+      `,
+    },
+  },
+  async ({ fs, spawn }) => {
+    await spawn('pnpm tailwindcss --input src/index.css --output dist/out.css --watch')
+
+    await fs.expectFileToContain('dist/out.css', [
+      //
+      candidate`text-primary`,
+      'color: blue',
+    ])
+
+    await fs.write('my-color.mjs', js`export default 'red'`)
+
+    await fs.expectFileToContain('dist/out.css', [
+      //
+      candidate`text-primary`,
+      'color: red',
+    ])
+  },
+)