Emit @keyframes in prefixed setup (#16850)

Closes #16829

We were only adding keyframes used in variables starting with
`--animate` (without adding a potential prefix).

## Test plan

- Added a unit test

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Ensure `not-*` does not remove `:is(…)` from variants ([#16825](https://github.com/tailwindlabs/tailwindcss/pull/16825))
+- Ensure `@keyframes` are correctly emitted when using a prefixed setup ([#16850](https://github.com/tailwindlabs/tailwindcss/pull/16850))
 
 ## [4.0.9] - 2025-02-25
 

packages/tailwindcss/src/ast.ts

@@ -429,7 +429,7 @@ export function optimizeAst(ast: AstNode[], designSystem: DesignSystem) {
         variableDependencies,
       )
       if (variableUsed) {
-        if (declaration.property.startsWith('--animate-')) {
+        if (declaration.property.startsWith(designSystem.theme.prefixKey('--animate-'))) {
           let parts = declaration.value!.split(/\s+/)
           for (let part of parts) usedKeyframeNames.add(part)
         }

packages/tailwindcss/src/index.test.ts

@@ -1613,6 +1613,73 @@ describe('Parsing theme values from CSS', () => {
     `)
   })
 
+  test('keyframes are generated when used in an animation within a prefixed setup', async () => {
+    expect(
+      await compileCss(
+        css`
+          @theme prefix(tw) {
+            --animate-foo: used 1s infinite;
+            --animate-bar: unused 1s infinite;
+
+            @keyframes used {
+              to {
+                opacity: 1;
+              }
+            }
+
+            @keyframes unused {
+              to {
+                opacity: 0;
+              }
+            }
+          }
+
+          @tailwind utilities;
+        `,
+        ['tw:animate-foo'],
+      ),
+    ).toMatchInlineSnapshot(`
+      ":root, :host {
+        --tw-animate-foo: used 1s infinite;
+      }
+
+      .tw\\:animate-foo {
+        animation: var(--tw-animate-foo);
+      }
+
+      @keyframes used {
+        to {
+          opacity: 1;
+        }
+      }"
+    `)
+  })
+
+  test('custom properties are generated when used from a CSS var with a prefixed setup', async () => {
+    expect(
+      await compileCss(
+        css`
+          @theme prefix(tw) {
+            --color-tomato: #e10c04;
+          }
+          @tailwind utilities;
+          .red {
+            color: var(--tw-color-tomato);
+          }
+        `,
+        [],
+      ),
+    ).toMatchInlineSnapshot(`
+      ":root, :host {
+        --tw-color-tomato: #e10c04;
+      }
+
+      .red {
+        color: var(--tw-color-tomato);
+      }"
+    `)
+  })
+
   // https://github.com/tailwindlabs/tailwindcss/issues/16374
   test('custom properties in keyframes preserved', async () => {
     expect(

packages/tailwindcss/src/theme.ts

@@ -117,12 +117,12 @@ export class Theme {
     if (!this.prefix) return this.values.entries()
 
     return Array.from(this.values, (entry) => {
-      entry[0] = this.#prefixKey(entry[0])
+      entry[0] = this.prefixKey(entry[0])
       return entry
     })
   }
 
-  #prefixKey(key: string) {
+  prefixKey(key: string) {
     if (!this.prefix) return key
     return `--${this.prefix}-${key.slice(2)}`
   }
@@ -190,7 +190,7 @@ export class Theme {
       fallback = value.value
     }
 
-    return `var(${escape(this.#prefixKey(themeKey))}${fallback ? `, ${fallback}` : ''})`
+    return `var(${escape(this.prefixKey(themeKey))}${fallback ? `, ${fallback}` : ''})`
   }
 
   markUsedVariable(themeKey: string) {