Don't consider the global important state in @apply

CHANGELOG.md

@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-- Nothing yet!
+### Fixed
+
+- Don't consider the global important state in `@apply` ([#18404](https://github.com/tailwindlabs/tailwindcss/pull/18404))
 
 ## [4.1.11] - 2025-06-26
 

packages/@tailwindcss-upgrade/src/codemods/template/signatures.ts

@@ -2,7 +2,7 @@ import { substituteAtApply } from '../../../../tailwindcss/src/apply'
 import { atRule, styleRule, toCss, walk, type AstNode } from '../../../../tailwindcss/src/ast'
 import { printArbitraryValue } from '../../../../tailwindcss/src/candidate'
 import * as SelectorParser from '../../../../tailwindcss/src/compat/selector-parser'
-import type { DesignSystem } from '../../../../tailwindcss/src/design-system'
+import { CompileAstFlags, type DesignSystem } from '../../../../tailwindcss/src/design-system'
 import { ThemeOptions } from '../../../../tailwindcss/src/theme'
 import { DefaultMap } from '../../../../tailwindcss/src/utils/default-map'
 import { isValidSpacingMultiplier } from '../../../../tailwindcss/src/utils/infer-data-type'
@@ -40,7 +40,16 @@ export const computeUtilitySignature = new DefaultMap<
       // Use `@apply` to normalize the selector to `.x`
       let ast: AstNode[] = [styleRule('.x', [atRule('@apply', utility)])]
 
-      temporarilyDisableThemeInline(designSystem, () => substituteAtApply(ast, designSystem))
+      temporarilyDisableThemeInline(designSystem, () => {
+        // There's separate utility caches for respect important vs not
+        // so we want to compile them both with `@theme inline` disabled
+        for (let candidate of designSystem.parseCandidate(utility)) {
+          designSystem.compileAstNodes(candidate, CompileAstFlags.None)
+          designSystem.compileAstNodes(candidate, CompileAstFlags.RespectImportant)
+        }
+
+        substituteAtApply(ast, designSystem)
+      })
 
       // We will be mutating the AST, so we need to clone it first to not affect
       // the original AST

packages/tailwindcss/src/apply.ts

@@ -176,6 +176,7 @@ export function substituteAtApply(ast: AstNode[], designSystem: DesignSystem) {
         // with.
         let candidates = Object.keys(candidateOffsets)
         let compiled = compileCandidates(candidates, designSystem, {
+          respectImportant: false,
           onInvalidCandidate: (candidate) => {
             // When using prefix, make sure prefix is used in candidate
             if (designSystem.theme.prefix && !candidate.startsWith(designSystem.theme.prefix)) {

packages/tailwindcss/src/compile.ts

@@ -9,7 +9,7 @@ import {
   type StyleRule,
 } from './ast'
 import { type Candidate, type Variant } from './candidate'
-import { type DesignSystem } from './design-system'
+import { CompileAstFlags, type DesignSystem } from './design-system'
 import GLOBAL_PROPERTY_ORDER from './property-order'
 import { asColor, type Utility } from './utilities'
 import { compare } from './utils/compare'
@@ -19,7 +19,10 @@ import type { Variants } from './variants'
 export function compileCandidates(
   rawCandidates: Iterable<string>,
   designSystem: DesignSystem,
-  { onInvalidCandidate }: { onInvalidCandidate?: (candidate: string) => void } = {},
+  {
+    onInvalidCandidate,
+    respectImportant,
+  }: { onInvalidCandidate?: (candidate: string) => void; respectImportant?: boolean } = {},
 ) {
   let nodeSorting = new Map<
     AstNode,
@@ -44,14 +47,20 @@ export function compileCandidates(
     matches.set(rawCandidate, candidates)
   }
 
+  let flags = CompileAstFlags.None
+
+  if (respectImportant || respectImportant === undefined) {
+    flags |= CompileAstFlags.RespectImportant
+  }
+
   let variantOrderMap = designSystem.getVariantOrder()
 
   // Create the AST
   for (let [rawCandidate, candidates] of matches) {
     let found = false
 
     for (let candidate of candidates) {
-      let rules = designSystem.compileAstNodes(candidate)
+      let rules = designSystem.compileAstNodes(candidate, flags)
       if (rules.length === 0) continue
 
       found = true
@@ -119,10 +128,16 @@ export function compileCandidates(
   }
 }
 
-export function compileAstNodes(candidate: Candidate, designSystem: DesignSystem) {
+export function compileAstNodes(
+  candidate: Candidate,
+  designSystem: DesignSystem,
+  flags: CompileAstFlags,
+) {
   let asts = compileBaseUtility(candidate, designSystem)
   if (asts.length === 0) return []
 
+  let respectImportant = Boolean(flags & CompileAstFlags.RespectImportant)
+
   let rules: {
     node: AstNode
     propertySort: {
@@ -136,7 +151,10 @@ export function compileAstNodes(candidate: Candidate, designSystem: DesignSystem
   for (let nodes of asts) {
     let propertySort = getPropertySort(nodes)
 
-    if (candidate.important || designSystem.important) {
+    // If the candidate itself is important then we want to always mark
+    // the utility as important. However, at a design system level we want
+    // to be able to opt-out when using things like `@apply`
+    if (candidate.important || (designSystem.important && respectImportant)) {
       applyImportant(nodes)
     }
 

packages/tailwindcss/src/design-system.ts

@@ -18,6 +18,11 @@ import { DefaultMap } from './utils/default-map'
 import { extractUsedVariables } from './utils/variables'
 import { Variants, createVariants } from './variants'
 
+export const enum CompileAstFlags {
+  None = 0,
+  RespectImportant = 1 << 0,
+}
+
 export type DesignSystem = {
   theme: Theme
   utilities: Utilities
@@ -34,7 +39,7 @@ export type DesignSystem = {
 
   parseCandidate(candidate: string): Readonly<Candidate>[]
   parseVariant(variant: string): Readonly<Variant> | null
-  compileAstNodes(candidate: Candidate): ReturnType<typeof compileAstNodes>
+  compileAstNodes(candidate: Candidate, flags?: CompileAstFlags): ReturnType<typeof compileAstNodes>
 
   printCandidate(candidate: Candidate): string
   printVariant(variant: Variant): string
@@ -57,26 +62,28 @@ export function buildDesignSystem(theme: Theme): DesignSystem {
     Array.from(parseCandidate(candidate, designSystem)),
   )
 
-  let compiledAstNodes = new DefaultMap<Candidate>((candidate) => {
-    let ast = compileAstNodes(candidate, designSystem)
-
-    // Arbitrary values (`text-[theme(--color-red-500)]`) and arbitrary
-    // properties (`[--my-var:theme(--color-red-500)]`) can contain function
-    // calls so we need evaluate any functions we find there that weren't in
-    // the source CSS.
-    try {
-      substituteFunctions(
-        ast.map(({ node }) => node),
-        designSystem,
-      )
-    } catch (err) {
-      // If substitution fails then the candidate likely contains a call to
-      // `theme()` that is invalid which may be because of incorrect usage,
-      // invalid arguments, or a theme key that does not exist.
-      return []
-    }
+  let compiledAstNodes = new DefaultMap<number>((flags) => {
+    return new DefaultMap<Candidate>((candidate) => {
+      let ast = compileAstNodes(candidate, designSystem, flags)
+
+      // Arbitrary values (`text-[theme(--color-red-500)]`) and arbitrary
+      // properties (`[--my-var:theme(--color-red-500)]`) can contain function
+      // calls so we need evaluate any functions we find there that weren't in
+      // the source CSS.
+      try {
+        substituteFunctions(
+          ast.map(({ node }) => node),
+          designSystem,
+        )
+      } catch (err) {
+        // If substitution fails then the candidate likely contains a call to
+        // `theme()` that is invalid which may be because of incorrect usage,
+        // invalid arguments, or a theme key that does not exist.
+        return []
+      }
 
-    return ast
+      return ast
+    })
   })
 
   let trackUsedVariables = new DefaultMap((raw) => {
@@ -134,8 +141,10 @@ export function buildDesignSystem(theme: Theme): DesignSystem {
     parseVariant(variant: string) {
       return parsedVariants.get(variant)
     },
-    compileAstNodes(candidate: Candidate) {
-      return compiledAstNodes.get(candidate)
+    compileAstNodes(candidate: Candidate, flags?: CompileAstFlags) {
+      flags ??= CompileAstFlags.RespectImportant
+
+      return compiledAstNodes.get(flags).get(candidate)
     },
 
     printCandidate(candidate: Candidate) {

packages/tailwindcss/src/index.test.ts

@@ -666,7 +666,7 @@ describe('@apply', () => {
   })
 
   // https://github.com/tailwindlabs/tailwindcss/issues/16935
-  it('should now swallow @utility declarations when @apply is used in nested rules', async () => {
+  it('should not swallow @utility declarations when @apply is used in nested rules', async () => {
     expect(
       await compileCss(
         css`
@@ -777,6 +777,41 @@ describe('@apply', () => {
       }"
     `)
   })
+
+  // https://github.com/tailwindlabs/tailwindcss/issues/18400
+  it('should ignore the design systems `important` flag when using @apply', async () => {
+    expect(
+      await compileCss(
+        css`
+          @import 'tailwindcss/utilities' important;
+          .flex-explicitly-important {
+            @apply flex!;
+          }
+          .flex-not-important {
+            @apply flex;
+          }
+        `,
+        ['flex'],
+        {
+          async loadStylesheet(_, base) {
+            return {
+              content: '@tailwind utilities;',
+              base,
+              path: '',
+            }
+          },
+        },
+      ),
+    ).toMatchInlineSnapshot(`
+      ".flex, .flex-explicitly-important {
+        display: flex !important;
+      }
+
+      .flex-not-important {
+        display: flex;
+      }"
+    `)
+  })
 })
 
 describe('arbitrary variants', () => {