diff --git a/.changeset/empty-goats-count.md b/.changeset/empty-goats-count.md
new file mode 100644
index 0000000000..08cca0d360
--- /dev/null
+++ b/.changeset/empty-goats-count.md
@@ -0,0 +1,5 @@
+---
+"stylelint": patch
+---
+
+Fixed: `property-no-unknown` false negatives for newer custom syntaxes
diff --git a/lib/utils/__tests__/isStandardSyntaxDeclaration.test.js b/lib/utils/__tests__/isStandardSyntaxDeclaration.test.js
index fcce7bba53..8c928ee3aa 100644
--- a/lib/utils/__tests__/isStandardSyntaxDeclaration.test.js
+++ b/lib/utils/__tests__/isStandardSyntaxDeclaration.test.js
@@ -166,6 +166,10 @@ describe('isStandardSyntaxDeclaration', () => {
 			true,
 		);
 	});
+
+	it('supports root-level declarations', () => {
+		expect(isStandardSyntaxDeclaration(decl('color: yellow;'))).toBe(true);
+	});
 });
 
 function decl(css, parser = postcss) {
diff --git a/lib/utils/isStandardSyntaxDeclaration.js b/lib/utils/isStandardSyntaxDeclaration.js
index 2f1f8b5787..e258a7d89f 100644
--- a/lib/utils/isStandardSyntaxDeclaration.js
+++ b/lib/utils/isStandardSyntaxDeclaration.js
@@ -1,14 +1,7 @@
 'use strict';
 
 const isScssVariable = require('./isScssVariable');
-const { isRoot, isRule } = require('./typeGuards');
-
-/**
- * @param {string} [lang]
- */
-function isStandardSyntaxLang(lang) {
-	return lang && (lang === 'css' || lang === 'custom-template' || lang === 'template-literal');
-}
+const { isRule } = require('./typeGuards');
 
 /**
  * Check whether a declaration is standard
@@ -19,18 +12,6 @@ module.exports = function isStandardSyntaxDeclaration(decl) {
 	const prop = decl.prop;
 	const parent = decl.parent;
 
-	// Declarations belong in a declaration block or standard CSS source
-	if (
-		parent &&
-		isRoot(parent) &&
-		parent.source &&
-		!isStandardSyntaxLang(
-			/** @type {import('postcss').Source & {lang?: string}} */ (parent.source).lang,
-		)
-	) {
-		return false;
-	}
-
 	// SCSS var; covers map and list declarations
 	if (isScssVariable(prop)) {
 		return false;