Add support for important in v4 (#14448)

This PR allows modifying utility output by wrapping all utility
declarations in a custom selector or marking all utility declarations as
`!important`. This is the v4 equivalent to the `important` option in the
`tailwind.config.js` file.

## Mark all utility declarations as `!important`

To add `!important` to all utility declarations, you add an `important`
flag after the `@import` statement for `tailwindcss` or
`tailwindcss/utilities`:

```css
/** Importing `tailwindcss` */
@import "tailwindcss" important;

/** Importing the utilities directly */
@import "tailwindcss/utilities" important;
```

Example utility output:

```css
.mx-auto {
  margin-left: auto !important;
  margin-right: auto !important;
}

.text-center {
  text-align: center !important;
}
```

This is equivalent to adding `important: true` to the
`tailwind.config.js` file — which is still supported for backwards
compatibility.

## Wrap all utility declarations in a custom selector

To nest all utilities in an `#app` selector you add `selector(#app)`
flag after the `@import` statement for `tailwindcss` or
`tailwindcss/utilities`:

```css
/** Importing `tailwindcss` */
@import "tailwindcss" selector(#app);

/** Importing the utilities directly */
@import "tailwindcss/utilities" selector(#app);
```

Example utility output:

```css
.mx-auto {
  #app & {
    margin-left: auto;
    margin-right: auto;
  }
}

.text-center {
  #app & {
    text-align: center;
  }
}
```

This is equivalent to adding `important: "#app"` to the
`tailwind.config.js` file — which is still supported for backwards
compatibility.

**This _does not_ bring back support for the `respectImportant` flag in
`addUtilities` / `matchUtilities`.**

Author: thecrypticace

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Expose timing information in debug mode ([#14553](https://github.com/tailwindlabs/tailwindcss/pull/14553))
 - Add support for `blocklist` in config files ([#14556](https://github.com/tailwindlabs/tailwindcss/pull/14556))
 - Add `color-scheme` utilities ([#14567](https://github.com/tailwindlabs/tailwindcss/pull/14567))
+- Add support wrapping utilities in a selector ([#14448](https://github.com/tailwindlabs/tailwindcss/pull/14448))
+- Add support marking all utilities as `!important` ([#14448](https://github.com/tailwindlabs/tailwindcss/pull/14448))
 - _Experimental_: Migrate `@import "tailwindcss/tailwind.css"` to `@import "tailwindcss"` ([#14514](https://github.com/tailwindlabs/tailwindcss/pull/14514))
 - _Experimental_: Migrate `@apply` utilities with the template codemods ([#14574](https://github.com/tailwindlabs/tailwindcss/pull/14574))
 - _Experimental_: Add template codemods for migrating variant order ([#14524](https://github.com/tailwindlabs/tailwindcss/pull/14524]))

packages/tailwindcss/src/compat/apply-compat-hooks.ts

@@ -1,4 +1,4 @@
-import { toCss, walk, type AstNode } from '../ast'
+import { rule, toCss, walk, WalkAction, type AstNode } from '../ast'
 import type { DesignSystem } from '../design-system'
 import type { Theme, ThemeKey } from '../theme'
 import { withAlpha } from '../utilities'
@@ -229,6 +229,29 @@ export async function applyCompatibilityHooks({
     designSystem.theme.prefix = resolvedConfig.prefix
   }
 
+  // If an important strategy has already been set in CSS don't override it
+  if (!designSystem.important && resolvedConfig.important === true) {
+    designSystem.important = true
+  }
+
+  if (typeof resolvedConfig.important === 'string') {
+    let wrappingSelector = resolvedConfig.important
+
+    walk(ast, (node, { replaceWith, parent }) => {
+      if (node.kind !== 'rule') return
+      if (node.selector !== '@tailwind utilities') return
+
+      // The AST node was already manually wrapped so there's nothing to do
+      if (parent?.kind === 'rule' && parent.selector === wrappingSelector) {
+        return WalkAction.Stop
+      }
+
+      replaceWith(rule(wrappingSelector, [node]))
+
+      return WalkAction.Stop
+    })
+  }
+
   for (let candidate of resolvedConfig.blocklist) {
     designSystem.invalidCandidates.add(candidate)
   }

packages/tailwindcss/src/compat/config.test.ts

@@ -1372,6 +1372,78 @@ test('a prefix must be letters only', async () => {
   )
 })
 
+test('important: `#app`', async () => {
+  let input = css`
+    @tailwind utilities;
+    @config "./config.js";
+
+    @utility custom {
+      color: red;
+    }
+  `
+
+  let compiler = await compile(input, {
+    loadModule: async (_, base) => ({
+      base,
+      module: { important: '#app' },
+    }),
+  })
+
+  expect(compiler.build(['underline', 'hover:line-through', 'custom'])).toMatchInlineSnapshot(`
+    "#app {
+      .custom {
+        color: red;
+      }
+      .underline {
+        text-decoration-line: underline;
+      }
+      .hover\\:line-through {
+        &:hover {
+          @media (hover: hover) {
+            text-decoration-line: line-through;
+          }
+        }
+      }
+    }
+    "
+  `)
+})
+
+test('important: true', async () => {
+  let input = css`
+    @tailwind utilities;
+    @config "./config.js";
+
+    @utility custom {
+      color: red;
+    }
+  `
+
+  let compiler = await compile(input, {
+    loadModule: async (_, base) => ({
+      base,
+      module: { important: true },
+    }),
+  })
+
+  expect(compiler.build(['underline', 'hover:line-through', 'custom'])).toMatchInlineSnapshot(`
+    ".custom {
+      color: red!important;
+    }
+    .underline {
+      text-decoration-line: underline!important;
+    }
+    .hover\\:line-through {
+      &:hover {
+        @media (hover: hover) {
+          text-decoration-line: line-through!important;
+        }
+      }
+    }
+    "
+  `)
+})
+
 test('blocklisted canddiates are not generated', async () => {
   let compiler = await compile(
     css`

packages/tailwindcss/src/compat/config/resolve-config.ts

@@ -29,6 +29,7 @@ interface ResolutionContext {
 let minimal: ResolvedConfig = {
   blocklist: [],
   prefix: '',
+  important: false,
   darkMode: null,
   theme: {},
   plugins: [],
@@ -69,6 +70,10 @@ export function resolveConfig(design: DesignSystem, files: ConfigFile[]): Resolv
     if ('blocklist' in config && config.blocklist !== undefined) {
       ctx.result.blocklist = config.blocklist ?? []
     }
+
+    if ('important' in config && config.important !== undefined) {
+      ctx.result.important = config.important ?? false
+    }
   }
 
   // Merge themes

packages/tailwindcss/src/compat/config/types.ts

@@ -87,3 +87,12 @@ export interface UserConfig {
 export interface ResolvedConfig {
   blocklist: string[]
 }
+
+// `important` support
+export interface UserConfig {
+  important?: boolean | string
+}
+
+export interface ResolvedConfig {
+  important: boolean | string
+}

packages/tailwindcss/src/compile.ts

@@ -140,7 +140,7 @@ export function compileAstNodes(candidate: Candidate, designSystem: DesignSystem
   for (let nodes of asts) {
     let propertySort = getPropertySort(nodes)
 
-    if (candidate.important) {
+    if (candidate.important || designSystem.important) {
       applyImportant(nodes)
     }
 

packages/tailwindcss/src/design-system.ts

@@ -15,6 +15,9 @@ export type DesignSystem = {
 
   invalidCandidates: Set<string>
 
+  // Whether to mark utility declarations as !important
+  important: boolean
+
   getClassOrder(classes: string[]): [string, bigint | null][]
   getClassList(): ClassEntry[]
   getVariants(): VariantEntry[]
@@ -48,6 +51,7 @@ export function buildDesignSystem(theme: Theme): DesignSystem {
     variants,
 
     invalidCandidates: new Set(),
+    important: false,
 
     candidatesToCss(classes: string[]) {
       let result: (string | null)[] = []

packages/tailwindcss/src/important.test.ts

@@ -0,0 +1,89 @@
+import { expect, test } from 'vitest'
+import { compile } from '.'
+
+const css = String.raw
+
+test('Utilities can be wrapped in a selector', async () => {
+  // This is the v4 equivalent of `important: "#app"` from v3
+  let input = css`
+    #app {
+      @tailwind utilities;
+    }
+  `
+
+  let compiler = await compile(input)
+
+  expect(compiler.build(['underline', 'hover:line-through'])).toMatchInlineSnapshot(`
+    "#app {
+      .underline {
+        text-decoration-line: underline;
+      }
+      .hover\\:line-through {
+        &:hover {
+          @media (hover: hover) {
+            text-decoration-line: line-through;
+          }
+        }
+      }
+    }
+    "
+  `)
+})
+
+test('Utilities can be marked with important', async () => {
+  // This is the v4 equivalent of `important: true` from v3
+  let input = css`
+    @import 'tailwindcss/utilities' important;
+  `
+
+  let compiler = await compile(input, {
+    loadStylesheet: async (id: string, base: string) => ({
+      base,
+      content: '@tailwind utilities;',
+    }),
+  })
+
+  expect(compiler.build(['underline', 'hover:line-through'])).toMatchInlineSnapshot(`
+    ".underline {
+      text-decoration-line: underline!important;
+    }
+    .hover\\:line-through {
+      &:hover {
+        @media (hover: hover) {
+          text-decoration-line: line-through!important;
+        }
+      }
+    }
+    "
+  `)
+})
+
+test('Utilities can be wrapped with a selector and marked as important', async () => {
+  // This does not have a direct equivalent in v3 but works as a consequence of
+  // the new APIs
+  let input = css`
+    @media important {
+      #app {
+        @tailwind utilities;
+      }
+    }
+  `
+
+  let compiler = await compile(input)
+
+  expect(compiler.build(['underline', 'hover:line-through'])).toMatchInlineSnapshot(`
+    "#app {
+      .underline {
+        text-decoration-line: underline!important;
+      }
+      .hover\\:line-through {
+        &:hover {
+          @media (hover: hover) {
+            text-decoration-line: line-through!important;
+          }
+        }
+      }
+    }
+    "
+  `)
+})

packages/tailwindcss/src/index.ts

@@ -76,6 +76,7 @@ async function parseCss(
   await substituteAtImports(ast, base, loadStylesheet)
 
   // Find all `@theme` declarations
+  let important: boolean | null = null
   let theme = new Theme()
   let customVariants: ((designSystem: DesignSystem) => void)[] = []
   let customUtilities: ((designSystem: DesignSystem) => void)[] = []
@@ -236,6 +237,35 @@ async function parseCss(
       return WalkAction.Skip
     }
 
+    if (node.selector.startsWith('@media')) {
+      let features = segment(node.selector.slice(6), ' ')
+      let shouldReplace = true
+
+      for (let i = 0; i < features.length; i++) {
+        let part = features[i]
+
+        // Drop instances of `@media important`
+        //
+        // We support `@import "tailwindcss" important` to mark all declarations
+        // in generated utilities as `!important`.
+        if (part === 'important') {
+          important = true
+          shouldReplace = true
+          features[i] = ''
+        }
+      }
+
+      let remaining = features.filter(Boolean).join(' ')
+
+      node.selector = `@media ${remaining}`
+
+      if (remaining.trim() === '' && shouldReplace) {
+        replaceWith(node.nodes)
+      }
+
+      return WalkAction.Skip
+    }
+
     if (node.selector !== '@theme' && !node.selector.startsWith('@theme ')) return
 
     let [themeOptions, themePrefix] = parseThemeOptions(node.selector)
@@ -288,6 +318,10 @@ async function parseCss(
 
   let designSystem = buildDesignSystem(theme)
 
+  if (important) {
+    designSystem.important = important
+  }
+
   // Apply hooks from backwards compatibility layer. This function takes a lot
   // of random arguments because it really just needs access to "the world" to
   // do whatever ungodly things it needs to do to make things backwards