diff --git a/CHANGELOG.md b/CHANGELOG.md
index d500e668..4c11c756 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 ### Added
 
 ### Changed
+- Throw exception with invalid CSS in debug mode (#1142)
 - Only support up to 69 atomic expressions in a selector (#1113)
 - Require `sabberworm/php-css-parser:^8.4.0` (#1134)
 - Upgrade to PHPUnit 9 (#1112)
diff --git a/src/Css/CssDocument.php b/src/Css/CssDocument.php
index 145510bd..4639d585 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
@@ -37,10 +38,30 @@ class CssDocument
 
     /**
      * @param string $css
+     * @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.
      */
-    public function __construct(string $css)
+    public function __construct(string $css, bool $debug)
     {
-        $this->sabberwormCssDocument = (new CssParser($css))->parse();
+        // CSS Parser currently throws exception with nested at-rules (like `@media`) in strict parsing mode
+        $parserSettings = ParserSettings::create()->withLenientParsing(!$debug || $this->hasNestedAtRule($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 function hasNestedAtRule(string $css): bool
+    {
+        return \preg_match('/@(?:media|supports|(?:-webkit-|-moz-|-ms-|-o-)?+(keyframes|document))\\b/', $css) === 1;
     }
 
     /**
diff --git a/src/CssInliner.php b/src/CssInliner.php
index bf0b9118..0e071292 100644
--- a/src/CssInliner.php
+++ b/src/CssInliner.php
@@ -179,7 +179,7 @@ public function inlineCss(string $css = ''): self
         if ($this->isStyleBlocksParsingEnabled) {
             $combinedCss .= $this->getCssFromAllStyleNodes();
         }
-        $parsedCss = new CssDocument($combinedCss);
+        $parsedCss = new CssDocument($combinedCss, $this->debug);
 
         $excludedNodes = $this->getNodesToExclude();
         $cssRules = $this->collateCssRules($parsedCss);
diff --git a/tests/Unit/Css/CssDocumentTest.php b/tests/Unit/Css/CssDocumentTest.php
index 75f91add..96688279 100644
--- a/tests/Unit/Css/CssDocumentTest.php
+++ b/tests/Unit/Css/CssDocumentTest.php
@@ -7,6 +7,8 @@
 use Pelago\Emogrifier\Css\CssDocument;
 use Pelago\Emogrifier\Tests\Support\Traits\AssertCss;
 use PHPUnit\Framework\TestCase;
+use Sabberworm\CSS\Parsing\UnexpectedEOFException;
+use Sabberworm\CSS\Parsing\UnexpectedTokenException;
 
 /**
  * @covers \Pelago\Emogrifier\Css\CssDocument
@@ -22,6 +24,11 @@ final class CssDocumentTest extends TestCase
         . '  font-family: "Foo Sans";' . "\n"
         . '  src: url("/foo-sans.woff2") format("woff2");' . "\n}";
 
+    private function createDebugSubject(string $css): CssDocument
+    {
+        return new CssDocument($css, true);
+    }
+
     /**
      * @test
      *
@@ -33,7 +40,7 @@ final class CssDocumentTest extends TestCase
     public function parsesSelector(string $selector): void
     {
         $css = $selector . '{ color: green; }';
-        $subject = new CssDocument($css);
+        $subject = $this->createDebugSubject($css);
 
         $result = $subject->getStyleRulesData([]);
 
@@ -47,7 +54,7 @@ public function parsesSelector(string $selector): void
     public function canParseMultipleSelectors(): void
     {
         $css = 'h1, h2 { color: green; }';
-        $subject = new CssDocument($css);
+        $subject = $this->createDebugSubject($css);
 
         $result = $subject->getStyleRulesData([]);
 
@@ -115,7 +122,7 @@ public function provideSelectorWithVariedWhitespace(): array
     public function parsesDeclarations(string $declarations): void
     {
         $css = 'p {' . $declarations . '}';
-        $subject = new CssDocument($css);
+        $subject = $this->createDebugSubject($css);
 
         $result = $subject->getStyleRulesData([]);
 
@@ -177,7 +184,7 @@ public function parsesAtMediaRule(string $mediaQuery): void
     {
         $atMediaAndQuery = '@media ' . $mediaQuery;
         $css = $atMediaAndQuery . ' { p { color: green; } }';
-        $subject = new CssDocument($css);
+        $subject = $this->createDebugSubject($css);
 
         $result = $subject->getStyleRulesData(['screen']);
 
@@ -218,7 +225,7 @@ public function parsesAtMediaRuleWithVariedWhitespace(
         $atMediaAndQuery = '@media' . $whitespaceAfterAtMedia . 'screen';
         $css = $atMediaAndQuery . $optionalWhitespaceWithinRule
             . '{' . $optionalWhitespaceWithinRule . 'p { color: green; }' . $optionalWhitespaceWithinRule . '}';
-        $subject = new CssDocument($css);
+        $subject = $this->createDebugSubject($css);
 
         $result = $subject->getStyleRulesData(['screen']);
 
@@ -252,7 +259,7 @@ public function provideVariedWhitespaceForAtMediaRule(): array
      */
     public function discardsMediaRuleWithTypeNotInAllowlist(string $mediaQuery): void
     {
-        $subject = new CssDocument('@media ' . $mediaQuery . ' { p { color: red; } }');
+        $subject = $this->createDebugSubject('@media ' . $mediaQuery . ' { p { color: red; } }');
 
         $result = $subject->getStyleRulesData(['screen']);
 
@@ -296,7 +303,9 @@ public function provideMediaQueryWithTvTypeAndVariedWhitespace(): array
      */
     public function parsesMultipleStyleRulesWithOtherCssBetween(string $cssBetween): void
     {
-        $subject = new CssDocument('p { color: green; }' . $cssBetween . '@media screen { h1 { color: green; } }');
+        $subject = $this->createDebugSubject(
+            'p { color: green; }' . $cssBetween . '@media screen { h1 { color: green; } }'
+        );
 
         $result = $subject->getStyleRulesData(['screen']);
 
@@ -314,7 +323,7 @@ public function parsesMultipleStyleRulesWithOtherCssBetween(string $cssBetween):
      */
     public function parsesMultipleStyleRulesWithOtherCssBefore(string $cssBefore): void
     {
-        $subject = new CssDocument(
+        $subject = $this->createDebugSubject(
             $cssBefore . 'p { color: green; } @media screen { h1 { color: green; } }'
         );
 
@@ -362,7 +371,7 @@ public function provideCssThatMustPrecedeStyleRules(): array
      */
     public function rendersValidNonConditionalAtRule(string $atRuleCss, string $cssBefore = ''): void
     {
-        $subject = new CssDocument($cssBefore . $atRuleCss);
+        $subject = $this->createDebugSubject($cssBefore . $atRuleCss);
 
         $result = $subject->renderNonConditionalAtRules();
 
@@ -403,7 +412,7 @@ public function provideValidNonConditionalAtRule(): array
      */
     public function rendersMultipleNonConditionalAtRules(string $cssBetween): void
     {
-        $subject = new CssDocument('@import "foo.css";' . $cssBetween . self::VALID_AT_FONT_FACE_RULE);
+        $subject = $this->createDebugSubject('@import "foo.css";' . $cssBetween . self::VALID_AT_FONT_FACE_RULE);
 
         $result = $subject->renderNonConditionalAtRules();
 
@@ -432,24 +441,56 @@ public function provideCssWithoutNonConditionalAtRules(): array
     /**
      * @test
      *
-     * @param string $css
+     * @dataProvider provideValidAtCharsetRule
+     * @dataProvider provideInvalidAtCharsetRuleWhichCausesException
+     * @dataProvider provideInvalidAtCharsetRuleWhichDoesNotCauseException
+     */
+    public function discardsValidOrInvalidAtCharsetRuleNotInDebugMode(string $css): void
+    {
+        $subject = new CssDocument($css, false);
+
+        $result = $subject->renderNonConditionalAtRules();
+
+        self::assertSame('', $result);
+    }
+
+    /**
+     * @test
      *
-     * @dataProvider provideValidAtCharsetRules
-     * @dataProvider provideInvalidAtCharsetRules
+     * @dataProvider provideValidAtCharsetRule
      */
-    public function discardsValidOrInvalidAtCharsetRule(string $css): void
+    public function discardsValidAtCharsetRuleInDebugMode(string $css): void
     {
-        $subject = new CssDocument($css);
+        $subject = $this->createDebugSubject($css);
 
         $result = $subject->renderNonConditionalAtRules();
 
         self::assertSame('', $result);
     }
 
+    /**
+     * @test
+     *
+     * @dataProvider provideInvalidAtCharsetRuleWhichCausesException
+     *
+     * Invalid `@charset` rules which do not currently cause an exception not yet tested.
+     */
+    public function throwsExceptionForInvalidAtCharsetRuleInDebugMode(string $css): void
+    {
+        $this->expectException(UnexpectedEOFException::class);
+
+        $this->createDebugSubject($css);
+
+        self::markTestSkipped(
+            'This test is disabled as currently the CSS parser does not throw an exception in all cases.'
+            . ' Discarding of invalid rules in non-debug mode is already covered by other tests.'
+        );
+    }
+
     /**
      * @return array<string, array{0: string}>
      */
-    public function provideValidAtCharsetRules(): array
+    public function provideValidAtCharsetRule(): array
     {
         return [
             'UTF-8' => ['@charset "UTF-8";'],
@@ -460,26 +501,33 @@ public function provideValidAtCharsetRules(): array
     /**
      * @return array<string, array{0: string}>
      */
-    public function provideInvalidAtCharsetRules(): array
+    public function provideInvalidAtCharsetRuleWhichCausesException(): array
+    {
+        return [
+            'with unquoted value' => ['@charset UTF-8;'],
+        ];
+    }
+
+    /**
+     * @return array<string, array{0: string}>
+     */
+    public function provideInvalidAtCharsetRuleWhichDoesNotCauseException(): array
     {
         return [
             'with uppercase identifier' => ['@CHARSET "UTF-8";'],
             'with extra space' => ['@charset  "UTF-8";'],
-            'with unquoted value' => ['@charset UTF-8;'],
         ];
     }
 
     /**
      * @test
      *
-     * @param string $atRuleCss
-     * @param string $cssBefore
-     *
-     * @dataProvider provideInvalidNonConditionalAtRule
+     * @dataProvider provideInvalidNonConditionalAtRuleWhichCausesException
+     * @dataProvider provideInvalidNonConditionalAtRuleWhichDoesNotCauseException
      */
-    public function notRendersInvalidNonConditionalAtRule(string $atRuleCss, string $cssBefore = ''): void
+    public function discardsInvalidNonConditionalAtRuleNotInDebugMode(string $atRuleCss, string $cssBefore = ''): void
     {
-        $subject = new CssDocument($cssBefore . $atRuleCss);
+        $subject = new CssDocument($cssBefore . $atRuleCss, false);
 
         $result = $subject->renderNonConditionalAtRules();
 
@@ -488,10 +536,37 @@ public function notRendersInvalidNonConditionalAtRule(string $atRuleCss, string
         self::assertStringNotContainsString($atAndRuleName, $result);
     }
 
+    /**
+     * @test
+     *
+     * @dataProvider provideInvalidNonConditionalAtRuleWhichCausesException
+     *
+     * Invalid non-conditional at-rules which do not currently cause an exception not yet tested.
+     */
+    public function throwsExceptionForInvalidNonConditionalAtRuleInDebugMode(
+        string $atRuleCss,
+        string $cssBefore = ''
+    ): void {
+        $this->expectException(UnexpectedTokenException::class);
+
+        $this->createDebugSubject($cssBefore . $atRuleCss);
+    }
+
+    /**
+     * @return array<string, array<int, string>>
+     */
+    public function provideInvalidNonConditionalAtRuleWhichCausesException(): array
+    {
+        return [
+            '`@charset` after style rule' => ['@charset "UTF-8";', 'p { color: red; }'],
+            '`@charset` after `@import` rule' => ['@charset "UTF-8";', '@import "foo.css";'],
+        ];
+    }
+
     /**
      * @return array<string, array<int, string>>
      */
-    public function provideInvalidNonConditionalAtRule(): array
+    public function provideInvalidNonConditionalAtRuleWhichDoesNotCauseException(): array
     {
         return [
             '`@font-face` without `font-family`' => ['
@@ -504,8 +579,6 @@ public function provideInvalidNonConditionalAtRule(): array
                   font-family: "Foo Sans";
                 }
             '],
-            '`@charset` after style rule' => ['@charset "UTF-8";', 'p { color: red; }'],
-            '`@charset` after `@import` rule' => ['@charset "UTF-8";', '@import "foo.css";'],
             '`@import` after style rule' => ['@import "foo.css";', 'p { color: red; }'],
             '`@import` after `@font-face` rule' => ['@import "foo.css";', self::VALID_AT_FONT_FACE_RULE],
         ];
@@ -516,7 +589,7 @@ public function provideInvalidNonConditionalAtRule(): array
      */
     public function notRendersAtMediaRuleInNonConditionalAtRules(): void
     {
-        $subject = new CssDocument('@media screen { p { color: red; } }');
+        $subject = $this->createDebugSubject('@media screen { p { color: red; } }');
 
         $result = $subject->renderNonConditionalAtRules();
 
diff --git a/tests/Unit/CssInlinerTest.php b/tests/Unit/CssInlinerTest.php
index 76d259af..07a63fa0 100644
--- a/tests/Unit/CssInlinerTest.php
+++ b/tests/Unit/CssInlinerTest.php
@@ -10,6 +10,7 @@
 use Pelago\Emogrifier\Tests\Support\Traits\AssertCss;
 use PHPUnit\Framework\Constraint\Constraint;
 use PHPUnit\Framework\TestCase;
+use Sabberworm\CSS\Parsing\UnexpectedTokenException;
 use Symfony\Component\CssSelector\CssSelectorConverter;
 use Symfony\Component\CssSelector\Exception\SyntaxErrorException;
 use TRegx\DataProvider\DataProviders;
@@ -1407,34 +1408,60 @@ static function (string $styleAttributeContent): Constraint {
     }
 
     /**
-     * @return string[][]
+     * @return array<string, array<int, string>>
      */
-    public function invalidDeclarationDataProvider(): array
+    public function provideInvalidDeclaration(): array
     {
         return [
             'missing dash in property name' => ['font weight: bold;'],
-            'invalid character in property name' => ['-9webkit-text-size-adjust:none;'],
             'missing :' => ['-webkit-text-size-adjust none'],
             'missing value' => ['-webkit-text-size-adjust :'],
         ];
     }
 
+    /**
+     * @return array<string, array<int, string>>
+     */
+    public function provideDeclarationWithInvalidPropertyName(): array
+    {
+        return [
+            'invalid character in property name' => ['-9webkit-text-size-adjust:none;'],
+        ];
+    }
+
     /**
      * @test
      *
      * @param string $cssDeclarationBlock the CSS declaration block (without the curly braces)
      *
-     * @dataProvider invalidDeclarationDataProvider
+     * @dataProvider provideInvalidDeclaration
+     * @dataProvider provideDeclarationWithInvalidPropertyName
      */
-    public function inlineCssDropsInvalidCssDeclaration(string $cssDeclarationBlock): void
+    public function inlineCssNotInDebugModeDropsInvalidCssDeclaration(string $cssDeclarationBlock): void
     {
-        $subject = $this->buildDebugSubject('<html></html>');
+        $subject = CssInliner::fromHtml('<html></html>');
 
         $subject->inlineCss('html {' . $cssDeclarationBlock . '}');
 
         self::assertStringContainsString('<html>', $subject->render());
     }
 
+    /**
+     * @test
+     *
+     * @param string $cssDeclarationBlock the CSS declaration block (without the curly braces)
+     *
+     * @dataProvider provideInvalidDeclaration
+     */
+    public function inlineCssInDebugModeForInvalidCssDeclarationThrowsException(string $cssDeclarationBlock): void
+    {
+        $this->expectException(UnexpectedTokenException::class);
+
+        $subject = $this->buildDebugSubject('<html></html>');
+
+        $subject->inlineCss('html {' . $cssDeclarationBlock . '}');
+    }
+
     /**
      * @test
      */
@@ -1557,10 +1584,7 @@ public function inlineCssRemovesStyleNodes(): void
      */
     public function inlineCssInDebugModeForInvalidCssSelectorThrowsException(): void
     {
-        // @see https://github.com/sabberworm/PHP-CSS-Parser/issues/347
-        self::markTestSkipped('This test is disabled as it currently is failing due to a bug in the CSS parser.');
-
-        $this->expectException(SyntaxErrorException::class);
+        $this->expectException(UnexpectedTokenException::class);
 
         $subject = CssInliner::fromHtml(
             '<html><style type="text/css">p{color:red;} <style data-x="1">html{cursor:text;}</style></html>'
@@ -3758,7 +3782,7 @@ public function copyUninlinableCssToStyleNodeHasNoSideEffects(): void
         $copyUninlinableCssToStyleNode->setAccessible(true);
 
         $domDocument = $subject->getDomDocument();
-        $cssDocument = new CssDocument('');
+        $cssDocument = new CssDocument('', true);
 
         $copyUninlinableCssToStyleNode->invoke($subject, $cssDocument);
         $expectedHtml = $subject->render();