From e7890429aaa9e1afbcb2584df25289e1c2eb43ab Mon Sep 17 00:00:00 2001
From: Sam Atkins <sam@ladybird.org>
Date: Tue, 12 Aug 2025 11:55:35 +0100
Subject: [PATCH] LibWeb/CSS: Add support for pseudo-classes taking `<an+b>#`

---
 .../LibWeb/CSS/Parser/SelectorParsing.cpp     | 33 +++++++++++++++++++
 Libraries/LibWeb/CSS/Selector.cpp             |  5 +++
 Libraries/LibWeb/Dump.cpp                     | 10 ++++++
 .../LibWeb/GenerateCSSPseudoClass.cpp         |  3 ++
 4 files changed, 51 insertions(+)

diff --git a/Libraries/LibWeb/CSS/Parser/SelectorParsing.cpp b/Libraries/LibWeb/CSS/Parser/SelectorParsing.cpp
index a9bbdcf6db6..2883b4c38fb 100644
--- a/Libraries/LibWeb/CSS/Parser/SelectorParsing.cpp
+++ b/Libraries/LibWeb/CSS/Parser/SelectorParsing.cpp
@@ -710,6 +710,37 @@ Parser::ParseErrorOr<Selector::SimpleSelector> Parser::parse_pseudo_simple_selec
             };
         };
 
+        auto parse_an_plus_b_list_selector = [this](auto pseudo_class, Vector<ComponentValue> const& function_values) -> ParseErrorOr<Selector::SimpleSelector> {
+            TokenStream tokens { function_values };
+            auto list = parse_a_comma_separated_list_of_component_values(tokens);
+            Vector<Selector::SimpleSelector::ANPlusBPattern> an_plus_b_patterns;
+
+            for (auto const& values : list) {
+                TokenStream pattern_tokens { values };
+                auto an_plus_b_pattern = parse_a_n_plus_b_pattern(pattern_tokens);
+                if (!an_plus_b_pattern.has_value()) {
+                    ErrorReporter::the().report(InvalidPseudoClassOrElementError {
+                        .name = MUST(String::formatted(":{}", pseudo_class_name(pseudo_class))),
+                        .value_string = pattern_tokens.dump_string(),
+                        .description = "Invalid An+B format."_string,
+                    });
+                    return ParseError::SyntaxError;
+                }
+                an_plus_b_patterns.append(an_plus_b_pattern.release_value());
+            }
+
+            tokens.discard_whitespace();
+            if (tokens.has_next_token())
+                return ParseError::SyntaxError;
+
+            return Selector::SimpleSelector {
+                .type = Selector::SimpleSelector::Type::PseudoClass,
+                .value = Selector::SimpleSelector::PseudoClassSelector {
+                    .type = pseudo_class,
+                    .an_plus_b_patterns = move(an_plus_b_patterns) }
+            };
+        };
+
         auto const& pseudo_function = pseudo_class_token.function();
         auto maybe_pseudo_class = pseudo_class_from_string(pseudo_function.name);
         if (!maybe_pseudo_class.has_value()) {
@@ -756,6 +787,8 @@ Parser::ParseErrorOr<Selector::SimpleSelector> Parser::parse_pseudo_simple_selec
         switch (metadata.parameter_type) {
         case PseudoClassMetadata::ParameterType::ANPlusB:
             return parse_an_plus_b_selector(pseudo_class, pseudo_function.value, false);
+        case PseudoClassMetadata::ParameterType::ANPlusBList:
+            return parse_an_plus_b_list_selector(pseudo_class, pseudo_function.value);
         case PseudoClassMetadata::ParameterType::ANPlusBOf:
             return parse_an_plus_b_selector(pseudo_class, pseudo_function.value, true);
         case PseudoClassMetadata::ParameterType::CompoundSelector: {
diff --git a/Libraries/LibWeb/CSS/Selector.cpp b/Libraries/LibWeb/CSS/Selector.cpp
index 34be49f512c..2d85ec69f8d 100644
--- a/Libraries/LibWeb/CSS/Selector.cpp
+++ b/Libraries/LibWeb/CSS/Selector.cpp
@@ -462,6 +462,11 @@ String Selector::SimpleSelector::serialize() const
                 // The result of serializing the value using the rules to serialize an <an+b> value.
                 s.append(pseudo_class.an_plus_b_patterns.first().serialize());
                 break;
+            case PseudoClassMetadata::ParameterType::ANPlusBList:
+                serialize_a_comma_separated_list(s, pseudo_class.an_plus_b_patterns, [](auto& builder, ANPlusBPattern const& an_plus_b) {
+                    builder.append(an_plus_b.serialize());
+                });
+                break;
             case PseudoClassMetadata::ParameterType::CompoundSelector:
             case PseudoClassMetadata::ParameterType::ForgivingSelectorList:
             case PseudoClassMetadata::ParameterType::ForgivingRelativeSelectorList:
diff --git a/Libraries/LibWeb/Dump.cpp b/Libraries/LibWeb/Dump.cpp
index 918331ee4d1..d103a8b7c16 100644
--- a/Libraries/LibWeb/Dump.cpp
+++ b/Libraries/LibWeb/Dump.cpp
@@ -575,6 +575,16 @@ void dump_selector(StringBuilder& builder, CSS::Selector const& selector, int in
                     builder.append(")"sv);
                     break;
                 }
+                case CSS::PseudoClassMetadata::ParameterType::ANPlusBList: {
+                    builder.append("([\n"sv);
+                    for (auto& an_plus_b : pseudo_class.an_plus_b_patterns) {
+                        indent(builder, indent_levels + 2);
+                        builder.appendff("(step={}, offset={})\n", an_plus_b.step_size, an_plus_b.offset);
+                    }
+                    indent(builder, indent_levels + 1);
+                    builder.append("])"sv);
+                    break;
+                }
                 case CSS::PseudoClassMetadata::ParameterType::CompoundSelector:
                 case CSS::PseudoClassMetadata::ParameterType::ForgivingSelectorList:
                 case CSS::PseudoClassMetadata::ParameterType::ForgivingRelativeSelectorList:
diff --git a/Meta/Lagom/Tools/CodeGenerators/LibWeb/GenerateCSSPseudoClass.cpp b/Meta/Lagom/Tools/CodeGenerators/LibWeb/GenerateCSSPseudoClass.cpp
index aed4abe6adc..1bf225a9d3c 100644
--- a/Meta/Lagom/Tools/CodeGenerators/LibWeb/GenerateCSSPseudoClass.cpp
+++ b/Meta/Lagom/Tools/CodeGenerators/LibWeb/GenerateCSSPseudoClass.cpp
@@ -70,6 +70,7 @@ struct PseudoClassMetadata {
     enum class ParameterType {
         None,
         ANPlusB,
+        ANPlusBList,
         ANPlusBOf,
         CompoundSelector,
         ForgivingSelectorList,
@@ -168,6 +169,8 @@ PseudoClassMetadata pseudo_class_metadata(PseudoClass pseudo_class)
         if (is_valid_as_function) {
             if (argument_string == "<an+b>"sv) {
                 parameter_type = "ANPlusB"_string;
+            } else if (argument_string == "<an+b>#"sv) {
+                parameter_type = "ANPlusBList"_string;
             } else if (argument_string == "<an+b-of>"sv) {
                 parameter_type = "ANPlusBOf"_string;
             } else if (argument_string == "<compound-selector>"sv) {