diff --git a/src/Css/CssDocument.php b/src/Css/CssDocument.php
index 3d792a8f..9c00785a 100644
--- a/src/Css/CssDocument.php
+++ b/src/Css/CssDocument.php
@@ -13,6 +13,7 @@
 use Sabberworm\CSS\Renderable as CssRenderable;
 use Sabberworm\CSS\RuleSet\DeclarationBlock as CssDeclarationBlock;
 use Sabberworm\CSS\RuleSet\RuleSet as CssRuleSet;
+use Sabberworm\CSS\Settings as ParserSettings;
 
 /**
  * Parses and stores a CSS document from a string of CSS, and provides methods to obtain the CSS in parts or as data
@@ -36,7 +37,7 @@ class CssDocument
     private $isImportRuleAllowed = true;
 
     /**
-     * @param string $css
+     * @param string $css {@see https://github.com/squizlabs/PHP_CodeSniffer/issues/3521}
      * @param bool $debug
      *        If this is `true`, an exception will be thrown if invalid CSS is encountered.
      *        Otherwise the parser will try to do the best it can.
@@ -44,17 +45,25 @@ class CssDocument
     public function __construct(string $css, bool $debug)
     {
         // CSS Parser currently throws exception with nested at-rules (like `@media`) in strict parsing mode
-        // @see https://github.com/sabberworm/PHP-CSS-Parser/issues/127
-        $parserSettings = \Sabberworm\CSS\Settings::create()->withLenientParsing(
-            !$debug ||
-            \preg_match('/@(?:media|supports|(?:-webkit-|-moz-|-ms-|-o-)?+(keyframes|document))\\b/', $css) === 1
-        );
+        $parserSettings = ParserSettings::create()->withLenientParsing(!$debug || static::hasNestedAtRule($css));
 
         // CSS Parser currently throws exception with non-empty whitespace-only CSS in strict parsing mode, so `trim()`
         // @see https://github.com/sabberworm/PHP-CSS-Parser/issues/349
         $this->sabberwormCssDocument = (new CssParser(\trim($css), $parserSettings))->parse();
     }
 
+    /**
+     * Tests if a string of CSS appears to contain an at-rule with nested rules
+     * (`@media`, `@supports`, `@keyframes`, `@document`,
+     * the latter two additionally with vendor prefixes that may commonly be used).
+     *
+     * @see https://github.com/sabberworm/PHP-CSS-Parser/issues/127
+     */
+    private static function hasNestedAtRule(string $css): bool
+    {
+        return \preg_match('/@(?:media|supports|(?:-webkit-|-moz-|-ms-|-o-)?+(keyframes|document))\\b/', $css) === 1;
+    }
+
     /**
      * Collates the media query, selectors and declarations for individual rules from the parsed CSS, in order.
      *
diff --git a/tests/Unit/Css/CssDocumentTest.php b/tests/Unit/Css/CssDocumentTest.php
index 5a099db3..bc7b18ad 100644
--- a/tests/Unit/Css/CssDocumentTest.php
+++ b/tests/Unit/Css/CssDocumentTest.php
@@ -24,6 +24,11 @@ final class CssDocumentTest extends TestCase
         . '  font-family: "Foo Sans";' . "\n"
         . '  src: url("/foo-sans.woff2") format("woff2");' . "\n}";
 
+    private static function createDebugSubject(string $css): CssDocument
+    {
+        return new CssDocument($css, true);
+    }
+
     /**
      * @test
      *
@@ -436,8 +441,6 @@ public function provideCssWithoutNonConditionalAtRules(): array
     /**
      * @test
      *
-     * @param string $css
-     *
      * @dataProvider provideValidAtCharsetRules
      * @dataProvider provideInvalidAtCharsetRules
      */
@@ -453,8 +456,6 @@ public function discardsValidOrInvalidAtCharsetRuleNotInDebugMode(string $css):
     /**
      * @test
      *
-     * @param string $css
-     *
      * @dataProvider provideValidAtCharsetRules
      */
     public function discardsValidAtCharsetRuleInDebugMode(string $css): void
@@ -469,8 +470,6 @@ public function discardsValidAtCharsetRuleInDebugMode(string $css): void
     /**
      * @test
      *
-     * @param string $css
-     *
      * @dataProvider provideInvalidAtCharsetRules
      */
     public function throwsExceptionForOrDiscardsInvalidAtCharsetRuleInDebugMode(string $css): void
@@ -513,9 +512,6 @@ public function provideInvalidAtCharsetRules(): array
     /**
      * @test
      *
-     * @param string $atRuleCss
-     * @param string $cssBefore
-     *
      * @dataProvider provideInvalidNonConditionalAtRule
      */
     public function discardsInvalidNonConditionalAtRuleNotInDebugMode(string $atRuleCss, string $cssBefore = ''): void
@@ -590,11 +586,6 @@ public function notRendersAtMediaRuleInNonConditionalAtRules(): void
         self::assertSame('', $result);
     }
 
-    private static function createDebugSubject(string $css): CssDocument
-    {
-        return new CssDocument($css, true);
-    }
-
     /**
      * Asserts that two strings are the same after `trim`ming both of them.
      *