LibWeb: Implement `:nth-[last-]child(n of foo)` syntax

In Selectors level 4, `:nth-child()` and `:nth-last-child()` can both
optionally take a selector-list argument. This selector-list acts as a
filter, so that only elements matching the list are counted. For
example, this means that the following are equivalent:

```css
:nth-child(2n+1 of p) {}
p:nth-of-type(2n+1) {}
```

5 files changed, 92 insertions, 25 deletions

diff --git a/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp b/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
index 73a1a46fe3..fec89e8252 100644
--- a/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
+++ b/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
@@ -571,15 +571,37 @@ Result<Selector::SimpleSelector, Parser::ParsingResult> Parser::parse_simple_sel
 
         } else if (pseudo_class_token.is_function()) {
 
-            auto parse_nth_child_pattern = [this](auto& simple_selector, auto& pseudo_function) -> bool {
+            auto parse_nth_child_pattern = [this](Selector::SimpleSelector& simple_selector, StyleFunctionRule const& pseudo_function, bool allow_of = false) -> bool {
                 auto function_values = TokenStream<StyleComponentValueRule>(pseudo_function.values());
-                auto nth_child_pattern = parse_a_n_plus_b_pattern(function_values);
+                auto nth_child_pattern = parse_a_n_plus_b_pattern(function_values, allow_of ? AllowTrailingTokens::Yes : AllowTrailingTokens::No);
                 if (nth_child_pattern.has_value()) {
                     simple_selector.pseudo_class.nth_child_pattern = nth_child_pattern.value();
                 } else {
                     dbgln_if(CSS_PARSER_DEBUG, "!!! Invalid format for {}", pseudo_function.name());
                     return false;
                 }
+                if (!allow_of)
+                    return true;
+
+                function_values.skip_whitespace();
+                if (!function_values.has_next_token())
+                    return true;
+
+                // Parse the `of <selector-list>` syntax
+                auto& maybe_of = function_values.next_token();
+                if (!(maybe_of.is(Token::Type::Ident) && maybe_of.token().ident().equals_ignoring_case("of"sv)))
+                    return false;
+
+                function_values.skip_whitespace();
+                auto selector_list = parse_a_selector_list(function_values);
+                if (selector_list.is_error())
+                    return false;
+
+                function_values.skip_whitespace();
+                if (function_values.has_next_token())
+                    return false;
+
+                simple_selector.pseudo_class.argument_selector_list = selector_list.value();
                 return true;
             };
 
@@ -606,11 +628,11 @@ Result<Selector::SimpleSelector, Parser::ParsingResult> Parser::parse_simple_sel
                 simple_selector.pseudo_class.argument_selector_list = not_selector.release_value();
             } else if (pseudo_function.name().equals_ignoring_case("nth-child"sv)) {
                 simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::NthChild;
-                if (!parse_nth_child_pattern(simple_selector, pseudo_function))
+                if (!parse_nth_child_pattern(simple_selector, pseudo_function, true))
                     return ParsingResult::SyntaxError;
             } else if (pseudo_function.name().equals_ignoring_case("nth-last-child"sv)) {
                 simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::NthLastChild;
-                if (!parse_nth_child_pattern(simple_selector, pseudo_function))
+                if (!parse_nth_child_pattern(simple_selector, pseudo_function, true))
                     return ParsingResult::SyntaxError;
             } else if (pseudo_function.name().equals_ignoring_case("nth-of-type"sv)) {
                 simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::NthOfType;
@@ -4129,7 +4151,7 @@ RefPtr<StyleValue> Parser::parse_css_value(StyleComponentValueRule const& compon
     return {};
 }
 
-Optional<Selector::SimpleSelector::ANPlusBPattern> Parser::parse_a_n_plus_b_pattern(TokenStream<StyleComponentValueRule>& values)
+Optional<Selector::SimpleSelector::ANPlusBPattern> Parser::parse_a_n_plus_b_pattern(TokenStream<StyleComponentValueRule>& values, AllowTrailingTokens allow_trailing_tokens)
 {
     int a = 0;
     int b = 0;
@@ -4145,7 +4167,7 @@ Optional<Selector::SimpleSelector::ANPlusBPattern> Parser::parse_a_n_plus_b_patt
     auto make_return_value = [&]() -> Optional<Selector::SimpleSelector::ANPlusBPattern> {
         // When we think we are done, but there are more non-whitespace tokens, then it's a parse error.
         values.skip_whitespace();
-        if (values.has_next_token()) {
+        if (values.has_next_token() && allow_trailing_tokens == AllowTrailingTokens::No) {
             if constexpr (CSS_PARSER_DEBUG) {
                 dbgln_if(CSS_PARSER_DEBUG, "Extra tokens at end of An+B value:");
                 values.dump_all_tokens();
diff --git a/Userland/Libraries/LibWeb/CSS/Parser/Parser.h b/Userland/Libraries/LibWeb/CSS/Parser/Parser.h
index 5039614029..12e5fd0f8c 100644
--- a/Userland/Libraries/LibWeb/CSS/Parser/Parser.h
+++ b/Userland/Libraries/LibWeb/CSS/Parser/Parser.h
@@ -157,7 +157,11 @@ private:
     template<typename T>
     RefPtr<Supports> parse_a_supports(TokenStream<T>&);
 
-    Optional<Selector::SimpleSelector::ANPlusBPattern> parse_a_n_plus_b_pattern(TokenStream<StyleComponentValueRule>&);
+    enum class AllowTrailingTokens {
+        No,
+        Yes
+    };
+    Optional<Selector::SimpleSelector::ANPlusBPattern> parse_a_n_plus_b_pattern(TokenStream<StyleComponentValueRule>&, AllowTrailingTokens = AllowTrailingTokens::No);
 
     template<typename T>
     [[nodiscard]] NonnullRefPtrVector<StyleRule> consume_a_list_of_rules(TokenStream<T>&, bool top_level);
diff --git a/Userland/Libraries/LibWeb/CSS/Selector.cpp b/Userland/Libraries/LibWeb/CSS/Selector.cpp
index 0957a0643f..1d8ab119f8 100644
--- a/Userland/Libraries/LibWeb/CSS/Selector.cpp
+++ b/Userland/Libraries/LibWeb/CSS/Selector.cpp
@@ -42,6 +42,21 @@ u32 Selector::specificity() const
     u32 classes = 0;
     u32 tag_names = 0;
 
+    auto count_specificity_of_most_complex_selector = [&](auto& selector_list) {
+        u32 max_selector_list_argument_specificity = 0;
+        for (auto const& complex_selector : selector_list) {
+            max_selector_list_argument_specificity = max(max_selector_list_argument_specificity, complex_selector.specificity());
+        }
+
+        u32 child_ids = (max_selector_list_argument_specificity & ids_mask) >> ids_shift;
+        u32 child_classes = (max_selector_list_argument_specificity & classes_mask) >> classes_shift;
+        u32 child_tag_names = (max_selector_list_argument_specificity & tag_names_mask) >> tag_names_shift;
+
+        ids += child_ids;
+        classes += child_classes;
+        tag_names += child_tag_names;
+    };
+
     for (auto& list : m_compound_selectors) {
         for (auto& simple_selector : list.simple_selectors) {
             switch (simple_selector.type) {
@@ -60,18 +75,16 @@ u32 Selector::specificity() const
                 case SimpleSelector::PseudoClass::Type::Not: {
                     // The specificity of an :is(), :not(), or :has() pseudo-class is replaced by the
                     // specificity of the most specific complex selector in its selector list argument.
-                    u32 max_selector_list_argument_specificity = 0;
-                    for (auto const& complex_selector : simple_selector.pseudo_class.argument_selector_list) {
-                        max_selector_list_argument_specificity = max(max_selector_list_argument_specificity, complex_selector.specificity());
-                    }
-
-                    u32 child_ids = (max_selector_list_argument_specificity & ids_mask) >> ids_shift;
-                    u32 child_classes = (max_selector_list_argument_specificity & classes_mask) >> classes_shift;
-                    u32 child_tag_names = (max_selector_list_argument_specificity & tag_names_mask) >> tag_names_shift;
-
-                    ids += child_ids;
-                    classes += child_classes;
-                    tag_names += child_tag_names;
+                    count_specificity_of_most_complex_selector(simple_selector.pseudo_class.argument_selector_list);
+                    break;
+                }
+                case SimpleSelector::PseudoClass::Type::NthChild:
+                case SimpleSelector::PseudoClass::Type::NthLastChild: {
+                    // Analogously, the specificity of an :nth-child() or :nth-last-child() selector
+                    // is the specificity of the pseudo class itself (counting as one pseudo-class selector)
+                    // plus the specificity of the most specific complex selector in its selector list argument (if any).
+                    ++classes;
+                    count_specificity_of_most_complex_selector(simple_selector.pseudo_class.argument_selector_list);
                     break;
                 }
                 case SimpleSelector::PseudoClass::Type::Where:
diff --git a/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp b/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp
index bcd0707e2f..8d73df25b5 100644
--- a/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp
+++ b/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp
@@ -180,16 +180,35 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
         if (!parent)
             return false;
 
+        auto matches_selector_list = [](CSS::SelectorList const& list, DOM::Element const& element) {
+            if (list.is_empty())
+                return true;
+            for (auto const& child_selector : list) {
+                if (matches(child_selector, element)) {
+                    return true;
+                }
+            }
+            return false;
+        };
+
         int index = 1;
         switch (pseudo_class.type) {
         case CSS::Selector::SimpleSelector::PseudoClass::Type::NthChild: {
-            for (auto* child = parent->first_child_of_type<DOM::Element>(); child && child != &element; child = child->next_element_sibling())
-                ++index;
+            if (!matches_selector_list(pseudo_class.argument_selector_list, element))
+                return false;
+            for (auto* child = parent->first_child_of_type<DOM::Element>(); child && child != &element; child = child->next_element_sibling()) {
+                if (matches_selector_list(pseudo_class.argument_selector_list, *child))
+                    ++index;
+            }
             break;
         }
         case CSS::Selector::SimpleSelector::PseudoClass::Type::NthLastChild: {
-            for (auto* child = parent->last_child_of_type<DOM::Element>(); child && child != &element; child = child->previous_element_sibling())
-                ++index;
+            if (!matches_selector_list(pseudo_class.argument_selector_list, element))
+                return false;
+            for (auto* child = parent->last_child_of_type<DOM::Element>(); child && child != &element; child = child->previous_element_sibling()) {
+                if (matches_selector_list(pseudo_class.argument_selector_list, *child))
+                    ++index;
+            }
             break;
         }
         case CSS::Selector::SimpleSelector::PseudoClass::Type::NthOfType: {
diff --git a/Userland/Libraries/LibWeb/Dump.cpp b/Userland/Libraries/LibWeb/Dump.cpp
index 7916a996d3..b1029cc36a 100644
--- a/Userland/Libraries/LibWeb/Dump.cpp
+++ b/Userland/Libraries/LibWeb/Dump.cpp
@@ -447,8 +447,17 @@ void dump_selector(StringBuilder& builder, CSS::Selector const& selector)
                         dump_selector(builder, selector);
                     builder.append("])");
                 } else if ((pseudo_class.type == CSS::Selector::SimpleSelector::PseudoClass::Type::NthChild)
-                    || (pseudo_class.type == CSS::Selector::SimpleSelector::PseudoClass::Type::NthLastChild)) {
-                    builder.appendff("(step={}, offset={})", pseudo_class.nth_child_pattern.step_size, pseudo_class.nth_child_pattern.offset);
+                    || (pseudo_class.type == CSS::Selector::SimpleSelector::PseudoClass::Type::NthLastChild)
+                    || (pseudo_class.type == CSS::Selector::SimpleSelector::PseudoClass::Type::NthOfType)
+                    || (pseudo_class.type == CSS::Selector::SimpleSelector::PseudoClass::Type::NthLastOfType)) {
+                    builder.appendff("(step={}, offset={}", pseudo_class.nth_child_pattern.step_size, pseudo_class.nth_child_pattern.offset);
+                    if (!pseudo_class.argument_selector_list.is_empty()) {
+                        builder.append(", selectors=[");
+                        for (auto const& child_selector : pseudo_class.argument_selector_list)
+                            dump_selector(builder, child_selector);
+                        builder.append("]");
+                    }
+                    builder.append(")");
                 }
             }