Prevent modifying CSS variables in plugins (#16103)

closes #16100

---------

Co-authored-by: Robin Malfait <[email protected]>
Co-authored-by: Philipp Spiess <[email protected]>

Author: 3 people

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Refactor gradient implementation to work around [prettier/prettier#17058](https://github.com/prettier/prettier/issues/17058) ([#16072](https://github.com/tailwindlabs/tailwindcss/pull/16072))
 - Vite: Ensure hot-reloading works with SolidStart setups ([#16052](https://github.com/tailwindlabs/tailwindcss/pull/16052))
 - Vite: Fix a crash when starting the development server in SolidStart setups ([#16052](https://github.com/tailwindlabs/tailwindcss/pull/16052))
+- Prevent camelCasing CSS custom properties added by JavaScript plugins ([#16103](https://github.com/tailwindlabs/tailwindcss/pull/16103))
 
 ## [4.0.1] - 2025-01-29
 

packages/tailwindcss/src/compat/plugin-api.test.ts

@@ -1534,6 +1534,38 @@ describe('addBase', () => {
       "
     `)
   })
+
+  test('does not modify CSS variables', async () => {
+    let input = css`
+      @plugin "my-plugin";
+    `
+
+    let compiler = await compile(input, {
+      loadModule: async () => ({
+        module: plugin(function ({ addBase }) {
+          addBase({
+            ':root': {
+              '--PascalCase': '1',
+              '--camelCase': '1',
+              '--UPPERCASE': '1',
+            },
+          })
+        }),
+        base: '/root',
+      }),
+    })
+
+    expect(compiler.build([])).toMatchInlineSnapshot(`
+      "@layer base {
+        :root {
+          --PascalCase: 1;
+          --camelCase: 1;
+          --UPPERCASE: 1;
+        }
+      }
+      "
+    `)
+  })
 })
 
 describe('addVariant', () => {

packages/tailwindcss/src/compat/plugin-api.ts

@@ -499,15 +499,18 @@ export function objectToAst(rules: CssInJs | CssInJs[]): AstNode[] {
 
   for (let [name, value] of entries) {
     if (typeof value !== 'object') {
-      if (!name.startsWith('--') && value === '@slot') {
-        ast.push(rule(name, [atRule('@slot')]))
-      } else {
+      if (!name.startsWith('--')) {
+        if (value === '@slot') {
+          ast.push(rule(name, [atRule('@slot')]))
+          continue
+        }
+
         // Convert camelCase to kebab-case:
         // https://github.com/postcss/postcss-js/blob/b3db658b932b42f6ac14ca0b1d50f50c4569805b/parser.js#L30-L35
         name = name.replace(/([A-Z])/g, '-$1').toLowerCase()
-
-        ast.push(decl(name, String(value)))
       }
+
+      ast.push(decl(name, String(value)))
     } else if (Array.isArray(value)) {
       for (let item of value) {
         if (typeof item === 'string') {