diff --git a/CHANGELOG.md b/CHANGELOG.md
index ba736074e..64c0ea940 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Prevent `@tailwindcss/vite` from crashing on every edit under Vite's experimental `bundledDev` mode ([#20379](https://github.com/tailwindlabs/tailwindcss/pull/20379))
 - Ensure `@tailwindcss/oxide` falls back to WASM on platforms without native bindings ([#20383](https://github.com/tailwindlabs/tailwindcss/pull/20383))
 - Detect classes in Ruby percent literals using angle brackets or custom delimiters (e.g. `%w<flex>`, `%w|flex|`), including in Slim and Haml templates ([#20387](https://github.com/tailwindlabs/tailwindcss/pull/20387))
+- Preserve whitespace in `--default(…)` values in custom functional utilities (e.g. `--default(box alphabetic)` no longer becomes `boxalphabetic`) ([#20392](https://github.com/tailwindlabs/tailwindcss/pull/20392))
 
 ## [4.3.3] - 2026-07-16
 
diff --git a/packages/tailwindcss/src/utilities.test.ts b/packages/tailwindcss/src/utilities.test.ts
index 9b514cda7..c9d4a5ff0 100644
--- a/packages/tailwindcss/src/utilities.test.ts
+++ b/packages/tailwindcss/src/utilities.test.ts
@@ -31000,6 +31000,34 @@ describe('custom utilities', () => {
       expect(await run(['example-123/foo'], input)).toEqual('')
     })
 
+    test('the value of `--default(…)` is emitted as-is', async () => {
+      let input = css`
+        @theme reference {
+          --text-box-trim-trim-both: trim-both;
+          --text-box-edge-cap: cap alphabetic;
+        }
+
+        @utility trim-* {
+          text-box: --value(--text-box-trim-*)
+            --modifier(--text-box-edge-*, --default(box alphabetic));
+        }
+
+        @tailwind utilities;
+      `
+
+      expect(await run(['trim-trim-both', 'trim-trim-both/cap'], input)).toMatchInlineSnapshot(`
+        "
+        .trim-trim-both {
+          text-box: var(--text-box-trim-trim-both, trim-both) box alphabetic;
+        }
+
+        .trim-trim-both\\/cap {
+          text-box: var(--text-box-trim-trim-both, trim-both) var(--text-box-edge-cap, cap alphabetic);
+        }
+        "
+      `)
+    })
+
     test('modifiers', async () => {
       let input = css`
         @theme reference {
diff --git a/packages/tailwindcss/src/utilities.ts b/packages/tailwindcss/src/utilities.ts
index 54b472c60..6c4a20cbc 100644
--- a/packages/tailwindcss/src/utilities.ts
+++ b/packages/tailwindcss/src/utilities.ts
@@ -6193,6 +6193,15 @@ export function createCssUtility(node: AtRule) {
 
           let args = segment(ValueParser.toCss(fn.nodes), ',')
           for (let [idx, arg] of args.entries()) {
+            arg = arg.trim()
+
+            // The value of `--default(…)` is emitted as-is, so it should not be
+            // normalized.
+            if (arg.startsWith('--default(')) {
+              args[idx] = arg
+              continue
+            }
+
             // Transform escaped `\\*` -> `*`
             arg = arg.replace(/\\\*/g, '*')