LibWeb: Add method for whether tree-counting function is allowed

Some contexts (e.g. descriptors, media conditions) don't allow tree
counting functions, this commit adds an easy way to check if the current
value context is one of those.

Libraries/LibWeb/CSS/Parser/MediaParsing.cpp

@@ -458,6 +458,8 @@ Optional<MediaFeatureValue> Parser::parse_media_feature_value(MediaFeatureID med
     {
         auto transaction = tokens.begin_transaction();
         auto value = [this](MediaFeatureID media_feature, TokenStream<ComponentValue>& tokens) -> Optional<MediaFeatureValue> {
+            auto context_guard = push_temporary_value_parsing_context(SpecialContext::MediaCondition);
+
             // One branch for each member of the MediaFeatureValueType enum:
             // Identifiers
             if (tokens.next_token().is(Token::Type::Ident)) {

Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -1694,6 +1694,21 @@ bool Parser::context_allows_quirky_length() const
     return unitless_length_allowed;
 }
 
+bool Parser::context_allows_tree_counting_functions() const
+{
+    for (auto context : m_value_context) {
+        if (context.has<DescriptorContext>())
+            return false;
+
+        if (auto const* special_context = context.get_pointer<SpecialContext>(); special_context && first_is_one_of(*special_context, SpecialContext::DOMMatrixInitString, SpecialContext::MediaCondition))
+            return false;
+
+        // TODO: Handle other contexts where tree counting functions are not allowed
+    }
+
+    return true;
+}
+
 Vector<ComponentValue> Parser::parse_as_list_of_component_values()
 {
     return parse_a_list_of_component_values(m_token_stream);

Libraries/LibWeb/CSS/Parser/Parser.h

@@ -77,6 +77,8 @@ struct DescriptorContext {
 enum SpecialContext : u8 {
     AngularColorStopList,
     CubicBezierFunctionXCoordinate,
+    DOMMatrixInitString,
+    MediaCondition,
     ShadowBlurRadius,
     StepsIntervalsJumpNone,
     StepsIntervalsNormal,
@@ -583,6 +585,7 @@ private:
         return ScopeGuard { [&] { m_value_context.take_last(); } };
     }
     bool context_allows_quirky_length() const;
+    bool context_allows_tree_counting_functions() const;
 
     Vector<RuleContext> m_rule_context;
     HashTable<FlyString> m_declared_namespaces;

Libraries/LibWeb/CSS/Parser/ValueParsing.cpp

@@ -4142,6 +4142,9 @@ RefPtr<CalculatedStyleValue const> Parser::parse_calculated_value(ComponentValue
                 case SpecialContext::TranslateZArgument:
                     // Percentages are disallowed for the Z axis
                     return CalculationContext {};
+                case SpecialContext::DOMMatrixInitString:
+                case SpecialContext::MediaCondition:
+                    return {};
                 }
                 VERIFY_NOT_REACHED();
             });

Libraries/LibWeb/Geometry/DOMMatrixReadOnly.cpp

@@ -949,7 +949,10 @@ WebIDL::ExceptionOr<ParsedMatrix> parse_dom_matrix_init_string(JS::Realm& realm,
     // 2. Parse transformList into parsedValue given the grammar for the CSS transform property.
     // The result will be a <transform-list>, the keyword none, or failure.
     // If parsedValue is failure, or any <transform-function> has <length> values without absolute length units, or any keyword other than none is used, then return failure. [CSS3-SYNTAX] [CSS3-TRANSFORMS]
-    auto transform_style_value = parse_css_value(CSS::Parser::ParsingParams {}, transform_list, CSS::PropertyID::Transform);
+    CSS::Parser::ParsingParams parsing_params {};
+    parsing_params.value_context = { CSS::Parser::SpecialContext::DOMMatrixInitString };
+
+    auto transform_style_value = parse_css_value(parsing_params, transform_list, CSS::PropertyID::Transform);
     if (!transform_style_value || (transform_style_value->is_keyword() && transform_style_value->to_keyword() != CSS::Keyword::None) || transform_style_value->is_unresolved())
         return WebIDL::SyntaxError::create(realm, "Failed to parse CSS transform string."_utf16);
     auto parsed_value = CSS::ComputedProperties::transformations_for_style_value(*transform_style_value);