From cbb8702143bd04b68253758dbc2c4a8fb49c0237 Mon Sep 17 00:00:00 2001
From: Kristiyan Kostadinov <crisbeto@abv.bg>
Date: Wed, 9 Sep 2026 08:59:21 +0200
Subject: [PATCH] fix(compiler): namespace `@property` declarations

The `@property` atrule allows users to define custom CSS variables. These changes update the compiler to account for when namespacing variables.

(cherry picked from commit 9b80d4ce9e00d50d471c1a1a98ab67ec53fbce5f)
---
 packages/compiler/src/shadow_css.ts           | 15 +--
 .../test/shadow_css/shadow_css_spec.ts        | 92 ++++++++++++++++++-
 2 files changed, 98 insertions(+), 9 deletions(-)

diff --git a/packages/compiler/src/shadow_css.ts b/packages/compiler/src/shadow_css.ts
index ba43560b382..cfa1376020a 100644
--- a/packages/compiler/src/shadow_css.ts
+++ b/packages/compiler/src/shadow_css.ts
@@ -1036,32 +1036,33 @@ const _cssColonInPlaceholderReGlobal = new RegExp(COLON_IN_PLACEHOLDER, 'g');
 
 // Matches any CSS variable name, defined by a double-hyphen followed by any valid ident.
 // https://www.w3.org/TR/css-syntax-3/#ident-token-diagram
-const _cssVariableRe = /(var\(\s*)?(--(?:[a-zA-Z0-9_-]|[^\x00-\x7F])+)(\s*:)?/g;
+const _cssVariableRe = /(var\(\s*|@property\s+)?(--(?:[a-zA-Z0-9_-]|[^\x00-\x7F])+)(\s*:)?/g;
 
 /**
  * Transforms CSS variables within a stylesheet to include a namespace placeholder.
  *
  * E.g. `--foo: bar;` becomes `--%NS%foo: bar;`
  * E.g. `color: var(--foo);` becomes `color: var(--%NS%foo);`
+ * E.g. `@property --foo` becomes `@property --%NS%foo`
  *
  * If a variable is prefixed with `--global--`, it is NOT namespaced and the prefix is removed.
  * E.g. `--global--mycolor: red;` becomes `--mycolor: red;`
  */
 export function namespaceCssVariables(cssText: string): string {
-  return cssText.replace(_cssVariableRe, (match, leadingVar, varName, trailingColon) => {
-    // Check for a leading `var(` or trailing `:` to approximate whether we're operating on a
-    // real CSS variable, not another piece of syntax that resembles it. For example, this
-    // guards against:
+  return cssText.replace(_cssVariableRe, (match, prefix, varName, trailingColon) => {
+    // Check for a leading `var(`, `@property`, or trailing `:` to approximate whether we're
+    // operating on a real CSS variable, not another piece of syntax that resembles it.
+    // For example, this guards against:
     // - `.foo--bar {}`
     // - `/* --foo */`
     // - `p { content: "--foo" }`
     // - `[data---bar] {}`
     // - `[data-status=foo--bar] {}`
     // etc.
-    if (!leadingVar && !trailingColon) {
+    if (!prefix && !trailingColon) {
       return match;
     }
-    return (leadingVar ?? '') + namespaceCssVariable(varName) + (trailingColon ?? '');
+    return (prefix ?? '') + namespaceCssVariable(varName) + (trailingColon ?? '');
   });
 }
 
diff --git a/packages/compiler/test/shadow_css/shadow_css_spec.ts b/packages/compiler/test/shadow_css/shadow_css_spec.ts
index a0cd582caf9..e1bac0f76a4 100644
--- a/packages/compiler/test/shadow_css/shadow_css_spec.ts
+++ b/packages/compiler/test/shadow_css/shadow_css_spec.ts
@@ -433,7 +433,7 @@ describe('ShadowCss', () => {
       const input = `
 .foo {
   border: var(--global--border-size) solid var(--border-color);
-  box-shadow: 
+  box-shadow:
     var(--shadow-1),
     var(--global--shadow-2),
     var(--shadow-3);
@@ -443,7 +443,7 @@ describe('ShadowCss', () => {
       const expected = `
 .foo {
   border: var(--border-size) solid var(--%NS%border-color);
-  box-shadow: 
+  box-shadow:
     var(--%NS%shadow-1),
     var(--shadow-2),
     var(--%NS%shadow-3);
@@ -597,5 +597,93 @@ p {
         'CSS variable "--global-" has a single hyphen after "--global". Use two hyphens ("--global--") to opt-out of namespacing.',
       );
     });
+
+    it('should namespace @property declarations', () => {
+      const input = `
+  @property --myColor {
+    syntax: "<color>";
+    inherits: true;
+    initial-value: rebeccapurple;
+  }
+
+  @property --myWidth {
+    syntax: "<length> | <percentage>";
+    inherits: true;
+    initial-value: 200px;
+  }
+
+  p {
+    background-color: var(--myColor);
+    width: var(--myWidth);
+    color: white;
+  }
+`.trim();
+
+      const expected = `
+  @property --%NS%myColor {
+    syntax: "<color>";
+    inherits: true;
+    initial-value: rebeccapurple;
+  }
+
+  @property --%NS%myWidth {
+    syntax: "<length> | <percentage>";
+    inherits: true;
+    initial-value: 200px;
+  }
+
+  p {
+    background-color: var(--%NS%myColor);
+    width: var(--%NS%myWidth);
+    color: white;
+  }
+`.trim();
+
+      expect(namespaceCssVariables(input)).toBe(expected);
+    });
+
+    it('should not namespace @property if --global-- is present', () => {
+      const input = `
+  @property --global--my-color {
+    syntax: "<color>";
+    inherits: true;
+    initial-value: rebeccapurple;
+  }
+
+  @property --global--my-width {
+    syntax: "<length> | <percentage>";
+    inherits: true;
+    initial-value: 200px;
+  }
+
+  p {
+    background-color: var(--global--my-color);
+    width: var(--global--my-width);
+    color: white;
+  }
+`.trim();
+
+      const expected = `
+  @property --my-color {
+    syntax: "<color>";
+    inherits: true;
+    initial-value: rebeccapurple;
+  }
+
+  @property --my-width {
+    syntax: "<length> | <percentage>";
+    inherits: true;
+    initial-value: 200px;
+  }
+
+  p {
+    background-color: var(--my-color);
+    width: var(--my-width);
+    color: white;
+  }
+`.trim();
+
+      expect(namespaceCssVariables(input)).toBe(expected);
+    });
   });
 });