Do not emit @keyframes in @theme reference (#16120)

This PR fixes na issue where `@keyframes` were emitted if they wre in a
`@theme
reference` and anothe `@theme {}` (that is not a reference) was present.

E.g.:

```css
@reference "tailwindcss";

@theme {
  /* ... */
}
```

Produces:
```css
:root, :host {
}
@keyframes spin {
  to {
    transform: rotate(360deg);
  }
}
@keyframes ping {
  75%, 100% {
    transform: scale(2);
    opacity: 0;
  }
}
@keyframes pulse {
  50% {
    opacity: 0.5;
  }
}
@keyframes bounce {
  0%, 100% {
    transform: translateY(-25%);
    animation-timing-function: cubic-bezier(0.8, 0, 1, 1);
  }
  50% {
    transform: none;
    animation-timing-function: cubic-bezier(0, 0, 0.2, 1);
  }
}
```

With this PR, the produced CSS looks like this instead:
```css
:root, :host {
}
```

Note: the empty `:root, :host` will be solved in a subsequent PR.

### Test plan

Added some unit tests, and a dedicated integration test.

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - 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))
+- Do not emit `@keyframes` in `@theme reference` ([#16120](https://github.com/tailwindlabs/tailwindcss/pull/16120))
 
 ## [4.0.1] - 2025-01-29
 

integrations/cli/index.test.ts

@@ -1196,3 +1196,65 @@ test(
     `)
   },
 )
+
+test(
+  '@theme reference should never emit values',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/cli": "workspace:^"
+          }
+        }
+      `,
+      'src/index.css': css`
+        @reference "tailwindcss";
+
+        .keep-me {
+          color: red;
+        }
+      `,
+    },
+  },
+  async ({ fs, spawn, expect }) => {
+    let process = await spawn(
+      `pnpm tailwindcss --input src/index.css --output dist/out.css --watch`,
+    )
+    await process.onStderr((m) => m.includes('Done in'))
+
+    expect(await fs.dumpFiles('./dist/*.css')).toMatchInlineSnapshot(`
+      "
+      --- ./dist/out.css ---
+      .keep-me {
+        color: red;
+      }
+      "
+    `)
+
+    await fs.write(
+      './src/index.css',
+      css`
+        @reference "tailwindcss";
+
+        /* Not a reference! */
+        @theme {
+          --color-pink: pink;
+        }
+
+        .keep-me {
+          color: red;
+        }
+      `,
+    )
+    expect(await fs.dumpFiles('./dist/*.css')).toMatchInlineSnapshot(`
+      "
+      --- ./dist/out.css ---
+      .keep-me {
+        color: red;
+      }
+      "
+    `)
+  },
+)

packages/tailwindcss/src/index.test.ts

@@ -1530,6 +1530,75 @@ describe('Parsing themes values from CSS', () => {
     `)
   })
 
+  test('`@keyframes` added in `@theme reference` should not be emitted', async () => {
+    return expect(
+      await compileCss(
+        css`
+          @theme reference {
+            --animate-foo: foo 1s infinite;
+
+            @keyframes foo {
+              0%,
+              100% {
+                color: red;
+              }
+              50% {
+                color: blue;
+              }
+            }
+          }
+          @tailwind utilities;
+        `,
+        ['animate-foo'],
+      ),
+    ).toMatchInlineSnapshot(`
+      ".animate-foo {
+        animation: var(--animate-foo);
+      }"
+    `)
+  })
+
+  test('`@keyframes` added in `@theme reference` should not be emitted, even if another `@theme` block exists', async () => {
+    return expect(
+      await compileCss(
+        css`
+          @theme reference {
+            --animate-foo: foo 1s infinite;
+
+            @keyframes foo {
+              0%,
+              100% {
+                color: red;
+              }
+              50% {
+                color: blue;
+              }
+            }
+          }
+
+          @theme {
+            --color-pink: pink;
+          }
+
+          @tailwind utilities;
+        `,
+        ['bg-pink', 'animate-foo'],
+      ),
+    ).toMatchInlineSnapshot(`
+      ":root, :host {
+        --color-pink: pink;
+      }
+
+      .animate-foo {
+        animation: var(--animate-foo);
+      }
+
+      .bg-pink {
+        background-color: var(--color-pink);
+      }"
+    `)
+  })
+
   test('theme values added as reference that override existing theme value suppress the output of the original theme value as a variable', async () => {
     expect(
       await compileCss(

packages/tailwindcss/src/index.ts

@@ -454,6 +454,12 @@ async function parseCss(
         // Collect `@keyframes` rules to re-insert with theme variables later,
         // since the `@theme` rule itself will be removed.
         if (child.kind === 'at-rule' && child.name === '@keyframes') {
+          // Do not track/emit `@keyframes`, if they are part of a `@theme reference`.
+          if (themeOptions & ThemeOptions.REFERENCE) {
+            replaceWith([])
+            return WalkAction.Skip
+          }
+
           theme.addKeyframes(child)
           replaceWith([])
           return WalkAction.Skip