diff --git a/Libraries/LibWeb/CSS/Parser/PropertyParsing.cpp b/Libraries/LibWeb/CSS/Parser/PropertyParsing.cpp
index 935827bb5f3..8c2a11a1f1e 100644
--- a/Libraries/LibWeb/CSS/Parser/PropertyParsing.cpp
+++ b/Libraries/LibWeb/CSS/Parser/PropertyParsing.cpp
@@ -895,12 +895,15 @@ RefPtr<CSSStyleValue> Parser::parse_counter_definitions_value(TokenStream<Compon
         CounterDefinition definition {};
 
         // <counter-name> | <reversed-counter-name>
-        auto& token = tokens.consume_a_token();
-        if (token.is(Token::Type::Ident)) {
-            definition.name = token.token().ident();
+        auto& token = tokens.next_token();
+
+        // A <counter-name> name cannot match the keyword none; such an identifier is invalid as a <counter-name>.
+        if (auto counter_name = parse_custom_ident_value(tokens, { { "none"sv } })) {
+            definition.name = counter_name->custom_ident();
             definition.is_reversed = false;
         } else if (allow_reversed == AllowReversed::Yes && token.is_function("reversed"sv)) {
             TokenStream function_tokens { token.function().value };
+            tokens.discard_a_token();
             function_tokens.discard_whitespace();
             auto& name_token = function_tokens.consume_a_token();
             if (!name_token.is(Token::Type::Ident))
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-lists/parsing/counter-increment-invalid.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-lists/parsing/counter-increment-invalid.txt
new file mode 100644
index 00000000000..4622d38240a
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-lists/parsing/counter-increment-invalid.txt
@@ -0,0 +1,18 @@
+Harness status: OK
+
+Found 13 tests
+
+13 Pass
+Pass	e.style['counter-increment'] = "none chapter" should not set the property value
+Pass	e.style['counter-increment'] = "reversed(none)" should not set the property value
+Pass	e.style['counter-increment'] = "reversed(chapter)" should not set the property value
+Pass	e.style['counter-increment'] = "3" should not set the property value
+Pass	e.style['counter-increment'] = "99 imagenum" should not set the property value
+Pass	e.style['counter-increment'] = "section -1, imagenum 99" should not set the property value
+Pass	e.style['counter-increment'] = "section 3.14" should not set the property value
+Pass	e.style['counter-increment'] = "inherit 1" should not set the property value
+Pass	e.style['counter-increment'] = "initial 1" should not set the property value
+Pass	e.style['counter-increment'] = "unset 1" should not set the property value
+Pass	e.style['counter-increment'] = "default 1" should not set the property value
+Pass	e.style['counter-increment'] = "revert 1" should not set the property value
+Pass	e.style['counter-increment'] = "revert-layer 1" should not set the property value
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-lists/parsing/counter-reset-invalid.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-lists/parsing/counter-reset-invalid.txt
new file mode 100644
index 00000000000..cb081caa800
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-lists/parsing/counter-reset-invalid.txt
@@ -0,0 +1,20 @@
+Harness status: OK
+
+Found 15 tests
+
+15 Pass
+Pass	e.style['counter-reset'] = "none chapter" should not set the property value
+Pass	e.style['counter-reset'] = "reversed(none)" should not set the property value
+Pass	e.style['counter-reset'] = "reversed(chapter) none" should not set the property value
+Pass	e.style['counter-reset'] = "3" should not set the property value
+Pass	e.style['counter-reset'] = "reversed(3)" should not set the property value
+Pass	e.style['counter-reset'] = "99 imagenum" should not set the property value
+Pass	e.style['counter-reset'] = "99 reversed(imagenum)" should not set the property value
+Pass	e.style['counter-reset'] = "section -1, imagenum 99" should not set the property value
+Pass	e.style['counter-reset'] = "section 3.14" should not set the property value
+Pass	e.style['counter-reset'] = "inherit 0" should not set the property value
+Pass	e.style['counter-reset'] = "initial 0" should not set the property value
+Pass	e.style['counter-reset'] = "unset 0" should not set the property value
+Pass	e.style['counter-reset'] = "default 0" should not set the property value
+Pass	e.style['counter-reset'] = "revert 0" should not set the property value
+Pass	e.style['counter-reset'] = "revert-layer 0" should not set the property value
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-lists/parsing/counter-set-invalid.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-lists/parsing/counter-set-invalid.txt
new file mode 100644
index 00000000000..d0cbfd0a611
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-lists/parsing/counter-set-invalid.txt
@@ -0,0 +1,18 @@
+Harness status: OK
+
+Found 13 tests
+
+13 Pass
+Pass	e.style['counter-set'] = "none chapter" should not set the property value
+Pass	e.style['counter-set'] = "reversed(none)" should not set the property value
+Pass	e.style['counter-set'] = "reversed(chapter)" should not set the property value
+Pass	e.style['counter-set'] = "3" should not set the property value
+Pass	e.style['counter-set'] = "99 imagenum" should not set the property value
+Pass	e.style['counter-set'] = "section -1, imagenum 99" should not set the property value
+Pass	e.style['counter-set'] = "section 3.14" should not set the property value
+Pass	e.style['counter-set'] = "inherit 2" should not set the property value
+Pass	e.style['counter-set'] = "initial 2" should not set the property value
+Pass	e.style['counter-set'] = "unset 2" should not set the property value
+Pass	e.style['counter-set'] = "default 2" should not set the property value
+Pass	e.style['counter-set'] = "revert 2" should not set the property value
+Pass	e.style['counter-set'] = "revert-layer 2" should not set the property value
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-lists/parsing/counter-increment-invalid.html b/Tests/LibWeb/Text/input/wpt-import/css/css-lists/parsing/counter-increment-invalid.html
new file mode 100644
index 00000000000..b1b2d606823
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-lists/parsing/counter-increment-invalid.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>CSS Lists: parsing counter-increment with invalid values</title>
+<link rel="help" href="https://drafts.csswg.org/css-lists-3/#propdef-counter-increment">
+<meta name="assert" content="counter-increment supports only the grammar '[ <counter-name> <integer>? ]+ | none'.">
+<script src="../../../resources/testharness.js"></script>
+<script src="../../../resources/testharnessreport.js"></script>
+<script src="../../../css/support/parsing-testcommon.js"></script>
+</head>
+<body>
+<script>
+test_invalid_value('counter-increment', 'none chapter');
+test_invalid_value('counter-increment', 'reversed(none)');
+test_invalid_value('counter-increment', 'reversed(chapter)');
+test_invalid_value('counter-increment', '3');
+test_invalid_value('counter-increment', '99 imagenum');
+test_invalid_value('counter-increment', 'section -1, imagenum 99');
+test_invalid_value('counter-increment', 'section 3.14');
+test_invalid_value('counter-increment', 'inherit 1');
+test_invalid_value('counter-increment', 'initial 1');
+test_invalid_value('counter-increment', 'unset 1');
+test_invalid_value('counter-increment', 'default 1');
+test_invalid_value('counter-increment', 'revert 1');
+test_invalid_value('counter-increment', 'revert-layer 1');
+</script>
+</body>
+</html>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-lists/parsing/counter-reset-invalid.html b/Tests/LibWeb/Text/input/wpt-import/css/css-lists/parsing/counter-reset-invalid.html
new file mode 100644
index 00000000000..d8b0367947e
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-lists/parsing/counter-reset-invalid.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>CSS Lists: parsing counter-reset with invalid values</title>
+<link rel="help" href="https://drafts.csswg.org/css-lists-3/#propdef-counter-reset">
+<meta name="assert" content="counter-reset supports only the grammar '[ <counter-name> <integer>? | <reversed-counter-name> <integer>? ]+ | none'.">
+<script src="../../../resources/testharness.js"></script>
+<script src="../../../resources/testharnessreport.js"></script>
+<script src="../../../css/support/parsing-testcommon.js"></script>
+</head>
+<body>
+<script>
+test_invalid_value('counter-reset', 'none chapter');
+test_invalid_value('counter-reset', 'reversed(none)');
+test_invalid_value('counter-reset', 'reversed(chapter) none');
+test_invalid_value('counter-reset', '3');
+test_invalid_value('counter-reset', 'reversed(3)');
+test_invalid_value('counter-reset', '99 imagenum');
+test_invalid_value('counter-reset', '99 reversed(imagenum)');
+test_invalid_value('counter-reset', 'section -1, imagenum 99');
+test_invalid_value('counter-reset', 'section 3.14');
+test_invalid_value('counter-reset', 'inherit 0');
+test_invalid_value('counter-reset', 'initial 0');
+test_invalid_value('counter-reset', 'unset 0');
+test_invalid_value('counter-reset', 'default 0');
+test_invalid_value('counter-reset', 'revert 0');
+test_invalid_value('counter-reset', 'revert-layer 0');
+</script>
+</body>
+</html>
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-lists/parsing/counter-set-invalid.html b/Tests/LibWeb/Text/input/wpt-import/css/css-lists/parsing/counter-set-invalid.html
new file mode 100644
index 00000000000..47307b98d6a
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-lists/parsing/counter-set-invalid.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>CSS Lists: parsing counter-set with invalid values</title>
+<link rel="help" href="https://drafts.csswg.org/css-lists-3/#propdef-counter-set">
+<meta name="assert" content="counter-set supports only the grammar '[ <counter-name> <integer>? ]+ | none'.">
+<script src="../../../resources/testharness.js"></script>
+<script src="../../../resources/testharnessreport.js"></script>
+<script src="../../../css/support/parsing-testcommon.js"></script>
+</head>
+<body>
+<script>
+test_invalid_value('counter-set', 'none chapter');
+test_invalid_value('counter-set', 'reversed(none)');
+test_invalid_value('counter-set', 'reversed(chapter)');
+test_invalid_value('counter-set', '3');
+test_invalid_value('counter-set', '99 imagenum');
+test_invalid_value('counter-set', 'section -1, imagenum 99');
+test_invalid_value('counter-set', 'section 3.14');
+test_invalid_value('counter-set', 'inherit 2');
+test_invalid_value('counter-set', 'initial 2');
+test_invalid_value('counter-set', 'unset 2');
+test_invalid_value('counter-set', 'default 2');
+test_invalid_value('counter-set', 'revert 2');
+test_invalid_value('counter-set', 'revert-layer 2');
+</script>
+</body>
+</html>