LibWeb: Parse CSS selectors according to the spec

The spec does not directly tell us how to parse selectors, so there are
likely some bugs here, but I've used the spec language where possible.
This is very much based on the previous selector parsing code.

Any parse error inside a selector makes the entire SelectorList
invalid, so nothing is returned.

Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -175,126 +175,249 @@ NonnullRefPtr<CSSStyleSheet> Parser::parse_as_stylesheet(TokenStream<T>& tokens)
     return stylesheet;
 }
 
-NonnullRefPtrVector<Selector> Parser::parse_a_selector()
+Optional<SelectorList> Parser::parse_a_selector()
 {
     return parse_a_selector(m_token_stream);
 }
 
 template<typename T>
-NonnullRefPtrVector<Selector> Parser::parse_a_selector(TokenStream<T>& tokens)
+Optional<SelectorList> Parser::parse_a_selector(TokenStream<T>& tokens)
 {
     dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_a_selector");
 
-    auto comma_separated_lists = parse_as_comma_separated_list_of_component_values(tokens);
+    auto selector_list = parse_a_selector_list(tokens);
-    NonnullRefPtrVector<Selector> selectors;
+    if (selector_list.has_value())
+        return selector_list;
 
-    for (auto& selector_parts : comma_separated_lists) {
+    return {};
-        auto stream = TokenStream(selector_parts);
-        auto selector = parse_single_selector(stream);
-        if (selector)
-            selectors.append(selector.release_nonnull());
-    }
-
-    return selectors;
 }
 
-NonnullRefPtrVector<Selector> Parser::parse_a_relative_selector()
+Optional<SelectorList> Parser::parse_a_relative_selector()
 {
     return parse_a_relative_selector(m_token_stream);
 }
 
 template<typename T>
-NonnullRefPtrVector<Selector> Parser::parse_a_relative_selector(TokenStream<T>& tokens)
+Optional<SelectorList> Parser::parse_a_relative_selector(TokenStream<T>& tokens)
 {
     dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_a_relative_selector");
 
+    auto selector_list = parse_a_relative_selector_list(tokens);
+    if (selector_list.has_value())
+        return selector_list;
+
+    return {};
+}
+
+template<typename T>
+Optional<SelectorList> Parser::parse_a_selector_list(TokenStream<T>& tokens)
+{
+    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_a_selector_list");
+
     auto comma_separated_lists = parse_as_comma_separated_list_of_component_values(tokens);
 
     NonnullRefPtrVector<Selector> selectors;
-
     for (auto& selector_parts : comma_separated_lists) {
         auto stream = TokenStream(selector_parts);
-        auto selector = parse_single_selector(stream, true);
+        auto selector = parse_complex_selector(stream, false);
         if (selector)
             selectors.append(selector.release_nonnull());
+        else
+            return {};
     }
 
+    if (selectors.is_empty())
+        return {};
+
     return selectors;
 }
 
 template<typename T>
-RefPtr<Selector> Parser::parse_single_selector(TokenStream<T>& tokens, bool is_relative)
+Optional<SelectorList> Parser::parse_a_relative_selector_list(TokenStream<T>& tokens)
 {
-    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_single_selector");
+    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_a_relative_selector_list");
 
-    // FIXME: Bring this all in line with the spec. https://www.w3.org/TR/selectors-4/
+    auto comma_separated_lists = parse_as_comma_separated_list_of_component_values(tokens);
 
-    Vector<Selector::CompoundSelector> selectors;
+    NonnullRefPtrVector<Selector> selectors;
-
+    for (auto& selector_parts : comma_separated_lists) {
-    auto check_for_eof_or_whitespace = [&](T& current_value) -> bool {
+        auto stream = TokenStream(selector_parts);
-        if (current_value.is(Token::Type::EndOfFile))
+        auto selector = parse_complex_selector(stream, true);
-            return true;
+        if (selector)
-
+            selectors.append(selector.release_nonnull());
-        if (current_value.is(Token::Type::Whitespace)) {
+        else
-            tokens.reconsume_current_input_token();
+            return {};
-            return true;
     }
-        return false;
+
+    if (selectors.is_empty())
+        return {};
+
+    return selectors;
+}
+
+RefPtr<Selector> Parser::parse_complex_selector(TokenStream<StyleComponentValueRule>& tokens, bool allow_starting_combinator)
+{
+    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_complex_selector");
+
+    Vector<Selector::CompoundSelector> compound_selectors;
+
+    auto first_selector = parse_compound_selector(tokens);
+    if (first_selector.is_error())
+        return {};
+    if (!allow_starting_combinator) {
+        if (first_selector.value().combinator != Selector::Combinator::Descendant)
+            return {};
+        first_selector.value().combinator = Selector::Combinator::None;
+    }
+    compound_selectors.append(first_selector.value());
+
+    while (tokens.has_next_token()) {
+        auto compound_selector = parse_compound_selector(tokens);
+        if (compound_selector.is_error()) {
+            if (compound_selector.error() == SelectorParsingResult::Done)
+                break;
+            else
+                return {};
+        }
+        compound_selectors.append(compound_selector.value());
+    }
+
+    if (compound_selectors.is_empty())
+        return {};
+
+    return Selector::create(move(compound_selectors));
+}
+
+Result<Selector::CompoundSelector, Parser::SelectorParsingResult> Parser::parse_compound_selector(TokenStream<StyleComponentValueRule>& tokens)
+{
+    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_compound_selector");
+
+    tokens.skip_whitespace();
+
+    auto combinator = parse_selector_combinator(tokens).value_or(Selector::Combinator::Descendant);
+
+    tokens.skip_whitespace();
+
+    Vector<Selector::SimpleSelector> simple_selectors;
+
+    while (tokens.has_next_token()) {
+        auto component = parse_simple_selector(tokens);
+        if (component.is_error()) {
+            if (component.error() == SelectorParsingResult::Done)
+                break;
+            else
+                return component.error();
+        }
+
+        simple_selectors.append(component.value());
+    }
+
+    if (simple_selectors.is_empty())
+        return SelectorParsingResult::Done;
+
+    return Selector::CompoundSelector { combinator, move(simple_selectors) };
+}
+
+Optional<Selector::Combinator> Parser::parse_selector_combinator(TokenStream<StyleComponentValueRule>& tokens)
+{
+    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_selector_combinator");
+
+    auto& current_value = tokens.next_token();
+    if (current_value.is(Token::Type::Delim)) {
+        auto delim = current_value.token().delim();
+        if (delim == ">"sv) {
+            return Selector::Combinator::ImmediateChild;
+        } else if (delim == "+"sv) {
+            return Selector::Combinator::NextSibling;
+        } else if (delim == "~"sv) {
+            return Selector::Combinator::SubsequentSibling;
+        } else if (delim == "|"sv) {
+            auto& next = tokens.peek_token();
+            if (next.is(Token::Type::EndOfFile))
+                return {};
+
+            if (next.is(Token::Type::Delim) && next.token().delim() == "|"sv) {
+                tokens.next_token();
+                return Selector::Combinator::Column;
+            }
+        }
+    }
+
+    tokens.reconsume_current_input_token();
+    return {};
+}
+
+Result<Selector::SimpleSelector, Parser::SelectorParsingResult> Parser::parse_simple_selector(TokenStream<StyleComponentValueRule>& tokens)
+{
+    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_simple_selector");
+
+    auto peek_token_ends_selector = [&]() -> bool {
+        auto& value = tokens.peek_token();
+        return (value.is(Token::Type::EndOfFile) || value.is(Token::Type::Whitespace) || value.is(Token::Type::Comma));
     };
 
-    auto parse_simple_selector = [&]() -> Optional<Selector::SimpleSelector> {
+    if (peek_token_ends_selector())
-        auto current_value = tokens.next_token();
+        return SelectorParsingResult::Done;
-        if (check_for_eof_or_whitespace(current_value))
-            return {};
 
-        Selector::SimpleSelector simple_selector;
+    auto& first_value = tokens.next_token();
-        // FIXME: Handle namespace prefixes.
 
-        if (current_value.is(Token::Type::Delim) && ((Token)current_value).delim() == "*") {
+    if (first_value.is(Token::Type::Delim) && first_value.token().delim() == "*"sv) {
-            simple_selector.type = Selector::SimpleSelector::Type::Universal;
+        return Selector::SimpleSelector {
-        } else if (current_value.is(Token::Type::Hash)) {
+            .type = Selector::SimpleSelector::Type::Universal
-            if (((Token)current_value).m_hash_type != Token::HashType::Id) {
+        };
-                dbgln("Selector contains hash token that is not an id: {}", current_value.to_debug_string());
+
-                return {};
+    } else if (first_value.is(Token::Type::Hash)) {
+        if (first_value.token().hash_type() != Token::HashType::Id) {
+            dbgln_if(CSS_PARSER_DEBUG, "Selector contains hash token that is not an id: {}", first_value.to_debug_string());
+            return SelectorParsingResult::SyntaxError;
         }
-            simple_selector.type = Selector::SimpleSelector::Type::Id;
+        return Selector::SimpleSelector {
-            simple_selector.value = ((Token)current_value).m_value.to_string();
+            .type = Selector::SimpleSelector::Type::Id,
-        } else if (current_value.is(Token::Type::Delim) && ((Token)current_value).delim() == ".") {
+            .value = first_value.token().hash_value()
-            current_value = tokens.next_token();
+        };
-            if (check_for_eof_or_whitespace(current_value))
-                return {};
 
-            if (!current_value.is(Token::Type::Ident)) {
+    } else if (first_value.is(Token::Type::Delim) && first_value.token().delim() == "."sv) {
-                dbgln("Expected an ident after '.', got: {}", current_value.to_debug_string());
+        if (peek_token_ends_selector())
-                return {};
+            return SelectorParsingResult::SyntaxError;
+
+        auto& class_name_value = tokens.next_token();
+        if (!class_name_value.is(Token::Type::Ident)) {
+            dbgln_if(CSS_PARSER_DEBUG, "Expected an ident after '.', got: {}", class_name_value.to_debug_string());
+            return SelectorParsingResult::SyntaxError;
         }
+        return Selector::SimpleSelector {
+            .type = Selector::SimpleSelector::Type::Class,
+            .value = class_name_value.token().ident()
+        };
 
-            simple_selector.type = Selector::SimpleSelector::Type::Class;
+    } else if (first_value.is(Token::Type::Ident)) {
-            simple_selector.value = current_value.token().ident().to_lowercase_string();
+        return Selector::SimpleSelector {
-        } else if (current_value.is(Token::Type::Ident)) {
+            .type = Selector::SimpleSelector::Type::TagName,
-            simple_selector.type = Selector::SimpleSelector::Type::TagName;
+            .value = first_value.token().ident()
-            simple_selector.value = current_value.token().ident().to_lowercase_string();
+        };
-        } else if (current_value.is_block() && current_value.block().is_square()) {
-            simple_selector.type = Selector::SimpleSelector::Type::Attribute;
 
-            auto& attribute = simple_selector.attribute;
+    } else if (first_value.is_block() && first_value.block().is_square()) {
-
+        auto& attribute_parts = first_value.block().values();
-            Vector<StyleComponentValueRule> const& attribute_parts = current_value.block().values();
 
         if (attribute_parts.is_empty()) {
-                dbgln("CSS attribute selector is empty!");
+            dbgln_if(CSS_PARSER_DEBUG, "CSS attribute selector is empty!");
-                return {};
+            return SelectorParsingResult::SyntaxError;
         }
 
         // FIXME: Handle namespace prefix for attribute name.
         auto& attribute_part = attribute_parts.first();
         if (!attribute_part.is(Token::Type::Ident)) {
-                dbgln("Expected ident for attribute name, got: '{}'", attribute_part.to_debug_string());
+            dbgln_if(CSS_PARSER_DEBUG, "Expected ident for attribute name, got: '{}'", attribute_part.to_debug_string());
-                return {};
+            return SelectorParsingResult::SyntaxError;
         }
 
-            attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::HasAttribute;
+        Selector::SimpleSelector simple_selector {
-            attribute.name = attribute_part.token().ident();
+            .type = Selector::SimpleSelector::Type::Attribute,
+            .attribute = {
+                .match_type = Selector::SimpleSelector::Attribute::MatchType::HasAttribute,
+                .name = attribute_part.token().ident(),
+            }
+        };
 
         if (attribute_parts.size() == 1)
             return simple_selector;
@@ -302,265 +425,214 @@ RefPtr<Selector> Parser::parse_single_selector(TokenStream<T>& tokens, bool is_r
         size_t attribute_index = 1;
         auto& delim_part = attribute_parts.at(attribute_index);
         if (!delim_part.is(Token::Type::Delim)) {
-                dbgln("Expected a delim for attribute comparison, got: '{}'", delim_part.to_debug_string());
+            dbgln_if(CSS_PARSER_DEBUG, "Expected a delim for attribute comparison, got: '{}'", delim_part.to_debug_string());
-                return {};
+            return SelectorParsingResult::SyntaxError;
         }
 
-            if (delim_part.token().delim() == "=") {
+        if (delim_part.token().delim() == "="sv) {
-                attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::ExactValueMatch;
+            simple_selector.attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::ExactValueMatch;
             attribute_index++;
         } else {
             attribute_index++;
             if (attribute_index >= attribute_parts.size()) {
-                    dbgln("Attribute selector ended part way through a match type.");
+                dbgln_if(CSS_PARSER_DEBUG, "Attribute selector ended part way through a match type.");
-                    return {};
+                return SelectorParsingResult::SyntaxError;
             }
 
             auto& delim_second_part = attribute_parts.at(attribute_index);
             if (!(delim_second_part.is(Token::Type::Delim) && delim_second_part.token().delim() == "=")) {
-                    dbgln("Expected a double delim for attribute comparison, got: '{}{}'", delim_part.to_debug_string(), delim_second_part.to_debug_string());
+                dbgln_if(CSS_PARSER_DEBUG, "Expected a double delim for attribute comparison, got: '{}{}'", delim_part.to_debug_string(), delim_second_part.to_debug_string());
-                    return {};
+                return SelectorParsingResult::SyntaxError;
             }
 
-                if (delim_part.token().delim() == "~") {
+            if (delim_part.token().delim() == "~"sv) {
-                    attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::ContainsWord;
+                simple_selector.attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::ContainsWord;
                 attribute_index++;
-                } else if (delim_part.token().delim() == "*") {
+            } else if (delim_part.token().delim() == "*"sv) {
-                    attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::ContainsString;
+                simple_selector.attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::ContainsString;
                 attribute_index++;
-                } else if (delim_part.token().delim() == "|") {
+            } else if (delim_part.token().delim() == "|"sv) {
-                    attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::StartsWithSegment;
+                simple_selector.attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::StartsWithSegment;
                 attribute_index++;
-                } else if (delim_part.token().delim() == "^") {
+            } else if (delim_part.token().delim() == "^"sv) {
-                    attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::StartsWithString;
+                simple_selector.attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::StartsWithString;
                 attribute_index++;
-                } else if (delim_part.token().delim() == "$") {
+            } else if (delim_part.token().delim() == "$"sv) {
-                    attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::EndsWithString;
+                simple_selector.attribute.match_type = Selector::SimpleSelector::Attribute::MatchType::EndsWithString;
                 attribute_index++;
             }
         }
 
         if (attribute_index >= attribute_parts.size()) {
-                dbgln("Attribute selector ended without a value to match.");
+            dbgln_if(CSS_PARSER_DEBUG, "Attribute selector ended without a value to match.");
-                return {};
+            return SelectorParsingResult::SyntaxError;
         }
 
         auto& value_part = attribute_parts.at(attribute_index);
         if (!value_part.is(Token::Type::Ident) && !value_part.is(Token::Type::String)) {
-                dbgln("Expected a string or ident for the value to match attribute against, got: '{}'", value_part.to_debug_string());
+            dbgln_if(CSS_PARSER_DEBUG, "Expected a string or ident for the value to match attribute against, got: '{}'", value_part.to_debug_string());
-                return {};
+            return SelectorParsingResult::SyntaxError;
         }
-            attribute.value = value_part.token().is(Token::Type::Ident) ? value_part.token().ident() : value_part.token().string();
+        simple_selector.attribute.value = value_part.token().is(Token::Type::Ident) ? value_part.token().ident() : value_part.token().string();
 
         // FIXME: Handle case-sensitivity suffixes. https://www.w3.org/TR/selectors-4/#attribute-case
-        } else if (current_value.is(Token::Type::Colon)) {
+        return simple_selector;
+
+    } else if (first_value.is(Token::Type::Colon)) {
+        if (peek_token_ends_selector())
+            return SelectorParsingResult::SyntaxError;
+
         bool is_pseudo = false;
-
+        if (tokens.peek_token().is(Token::Type::Colon)) {
-            current_value = tokens.next_token();
-            if (check_for_eof_or_whitespace(current_value))
-                return {};
-
-            if (current_value.is(Token::Type::Colon)) {
             is_pseudo = true;
-                current_value = tokens.next_token();
+            tokens.next_token();
-                if (check_for_eof_or_whitespace(current_value))
+            if (peek_token_ends_selector())
-                    return {};
+                return SelectorParsingResult::SyntaxError;
         }
 
         if (is_pseudo) {
-                auto pseudo_name = ((Token)current_value).ident();
+            Selector::SimpleSelector simple_selector {
-                simple_selector.type = Selector::SimpleSelector::Type::PseudoElement;
+                .type = Selector::SimpleSelector::Type::PseudoElement
+            };
 
-                if (pseudo_name.equals_ignoring_case("before")) {
+            auto& name_token = tokens.next_token();
-                    simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::Before;
+            if (!name_token.is(Token::Type::Ident)) {
-                } else if (pseudo_name.equals_ignoring_case("after")) {
+                dbgln_if(CSS_PARSER_DEBUG, "Expected an ident for pseudo-element, got: '{}'", name_token.to_debug_string());
+                return SelectorParsingResult::SyntaxError;
+            }
+
+            auto pseudo_name = name_token.token().ident();
+
+            if (pseudo_name.equals_ignoring_case("after")) {
                 simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::After;
-                } else if (pseudo_name.equals_ignoring_case("first-line")) {
+            } else if (pseudo_name.equals_ignoring_case("before")) {
-                    simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::FirstLine;
+                simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::Before;
             } else if (pseudo_name.equals_ignoring_case("first-letter")) {
                 simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::FirstLetter;
+            } else if (pseudo_name.equals_ignoring_case("first-line")) {
+                simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::FirstLine;
             } else {
-                    return {};
+                dbgln_if(CSS_PARSER_DEBUG, "Unrecognized pseudo-element: '{}'", pseudo_name);
+                return SelectorParsingResult::SyntaxError;
             }
 
             return simple_selector;
         }
 
-            auto& pseudo_class = simple_selector.pseudo_class;
+        if (peek_token_ends_selector())
+            return SelectorParsingResult::SyntaxError;
 
-            current_value = tokens.next_token();
+        auto& pseudo_class_token = tokens.next_token();
-            if (check_for_eof_or_whitespace(current_value))
+        Selector::SimpleSelector simple_selector {
-                return {};
+            .type = Selector::SimpleSelector::Type::PseudoClass
+        };
 
-            simple_selector.type = Selector::SimpleSelector::Type::PseudoClass;
+        if (pseudo_class_token.is(Token::Type::Ident)) {
-            if (current_value.is(Token::Type::Ident)) {
+            auto pseudo_name = pseudo_class_token.token().ident();
-                auto pseudo_name = ((Token)current_value).ident();
+            if (pseudo_name.equals_ignoring_case("active")) {
-                if (pseudo_name.equals_ignoring_case("link")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Active;
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Link;
-                } else if (pseudo_name.equals_ignoring_case("visited")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Visited;
-                } else if (pseudo_name.equals_ignoring_case("active")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Active;
-                } else if (pseudo_name.equals_ignoring_case("hover")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Hover;
-                } else if (pseudo_name.equals_ignoring_case("focus")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Focus;
-                } else if (pseudo_name.equals_ignoring_case("first-child")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::FirstChild;
-                } else if (pseudo_name.equals_ignoring_case("last-child")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::LastChild;
-                } else if (pseudo_name.equals_ignoring_case("only-child")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::OnlyChild;
-                } else if (pseudo_name.equals_ignoring_case("empty")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Empty;
-                } else if (pseudo_name.equals_ignoring_case("root")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Root;
-                } else if (pseudo_name.equals_ignoring_case("first-of-type")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::FirstOfType;
-                } else if (pseudo_name.equals_ignoring_case("last-of-type")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::LastOfType;
-                } else if (pseudo_name.equals_ignoring_case("before")) {
-                    simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::Before;
-                } else if (pseudo_name.equals_ignoring_case("after")) {
-                    simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::After;
-                } else if (pseudo_name.equals_ignoring_case("disabled")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Disabled;
-                } else if (pseudo_name.equals_ignoring_case("enabled")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Enabled;
             } else if (pseudo_name.equals_ignoring_case("checked")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Checked;
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Checked;
-                } else if (pseudo_name.equals_ignoring_case("before")) {
+            } else if (pseudo_name.equals_ignoring_case("disabled")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Disabled;
+            } else if (pseudo_name.equals_ignoring_case("empty")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Empty;
+            } else if (pseudo_name.equals_ignoring_case("enabled")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Enabled;
+            } else if (pseudo_name.equals_ignoring_case("first-child")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::FirstChild;
+            } else if (pseudo_name.equals_ignoring_case("first-of-type")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::FirstOfType;
+            } else if (pseudo_name.equals_ignoring_case("focus")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Focus;
+            } else if (pseudo_name.equals_ignoring_case("hover")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Hover;
+            } else if (pseudo_name.equals_ignoring_case("last-child")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::LastChild;
+            } else if (pseudo_name.equals_ignoring_case("last-of-type")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::LastOfType;
+            } else if (pseudo_name.equals_ignoring_case("link")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Link;
+            } else if (pseudo_name.equals_ignoring_case("only-child")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::OnlyChild;
+            } else if (pseudo_name.equals_ignoring_case("root")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Root;
+            } else if (pseudo_name.equals_ignoring_case("visited")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Visited;
+
+            } else if (pseudo_name.equals_ignoring_case("after")) {
                 // Single-colon syntax allowed for compatibility. https://www.w3.org/TR/selectors/#pseudo-element-syntax
                 simple_selector.type = Selector::SimpleSelector::Type::PseudoElement;
-                    simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::Before;
-                } else if (pseudo_name.equals_ignoring_case("after")) {
-                    // See :before
-                    simple_selector.type = Selector::SimpleSelector::Type::PseudoElement;
                 simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::After;
-                } else if (pseudo_name.equals_ignoring_case("first-line")) {
+            } else if (pseudo_name.equals_ignoring_case("before")) {
-                    // See :before
+                // See :after
                 simple_selector.type = Selector::SimpleSelector::Type::PseudoElement;
-                    simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::FirstLine;
+                simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::Before;
             } else if (pseudo_name.equals_ignoring_case("first-letter")) {
-                    // See :before
+                // See :after
                 simple_selector.type = Selector::SimpleSelector::Type::PseudoElement;
                 simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::FirstLetter;
+            } else if (pseudo_name.equals_ignoring_case("first-line")) {
+                // See :after
+                simple_selector.type = Selector::SimpleSelector::Type::PseudoElement;
+                simple_selector.pseudo_element = Selector::SimpleSelector::PseudoElement::FirstLine;
             } else {
-                    dbgln("Unknown pseudo class: '{}'", pseudo_name);
+                dbgln_if(CSS_PARSER_DEBUG, "Unknown pseudo class: '{}'", pseudo_name);
-                    return simple_selector;
+                return SelectorParsingResult::SyntaxError;
             }
-            } else if (current_value.is(Token::Type::Function)) {
+
-                auto& pseudo_function = current_value.function();
+            return simple_selector;
-                if (pseudo_function.name().equals_ignoring_case("nth-child")) {
+
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::NthChild;
+        } else if (pseudo_class_token.is_function()) {
+
+            auto& pseudo_function = pseudo_class_token.function();
+            if (pseudo_function.name().equals_ignoring_case("not")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Not;
+                auto function_token_stream = TokenStream(pseudo_function.values());
+                auto not_selector = parse_a_selector(function_token_stream);
+                if (!not_selector.has_value()) {
+                    dbgln_if(CSS_PARSER_DEBUG, "Invalid selector in :not() clause");
+                    return SelectorParsingResult::SyntaxError;
+                }
+                simple_selector.pseudo_class.not_selector = not_selector.value();
+            } else if (pseudo_function.name().equals_ignoring_case("nth-child")) {
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::NthChild;
                 auto function_values = TokenStream<StyleComponentValueRule>(pseudo_function.values());
-                    auto nth_child_pattern = parse_nth_child_pattern(function_values);
+                auto nth_child_pattern = parse_a_n_plus_b_pattern(function_values);
                 if (nth_child_pattern.has_value()) {
-                        pseudo_class.nth_child_pattern = nth_child_pattern.value();
+                    simple_selector.pseudo_class.nth_child_pattern = nth_child_pattern.value();
                 } else {
-                        dbgln("Invalid nth-child format");
+                    dbgln_if(CSS_PARSER_DEBUG, "!!! Invalid nth-child format");
-                        return {};
+                    function_values.dump_all_tokens();
+                    return SelectorParsingResult::SyntaxError;
                 }
             } else if (pseudo_function.name().equals_ignoring_case("nth-last-child")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::NthLastChild;
+                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::NthLastChild;
                 auto function_values = TokenStream<StyleComponentValueRule>(pseudo_function.values());
-                    auto nth_child_pattern = parse_nth_child_pattern(function_values);
+                auto nth_child_pattern = parse_a_n_plus_b_pattern(function_values);
                 if (nth_child_pattern.has_value()) {
-                        pseudo_class.nth_child_pattern = nth_child_pattern.value();
+                    dbgln("+++ Got An+B pattern: '{}'", nth_child_pattern.value().to_string());
+                    simple_selector.pseudo_class.nth_child_pattern = nth_child_pattern.value();
                 } else {
-                        dbgln("Invalid nth-child format");
+                    dbgln_if(CSS_PARSER_DEBUG, "!!! Invalid nth-child format");
-                        return {};
+                    function_values.dump_all_tokens();
-                    }
+                    return SelectorParsingResult::SyntaxError;
-                } else if (pseudo_function.name().equals_ignoring_case("not")) {
-                    pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Not;
-                    auto function_token_stream = TokenStream(pseudo_function.values());
-                    pseudo_class.not_selector = parse_a_selector(function_token_stream);
-                } else {
-                    dbgln("Unknown pseudo class: '{}'()", pseudo_function.name());
-                    return {};
                 }
             } else {
-                dbgln("Unexpected Block in pseudo-class name, expected a function or identifier. '{}'", current_value.to_debug_string());
+                dbgln_if(CSS_PARSER_DEBUG, "Unknown pseudo class: '{}'()", pseudo_function.name());
-                return {};
+                return SelectorParsingResult::SyntaxError;
-            }
-        } else {
-            dbgln("Invalid simple selector!");
-            return {};
             }
 
             return simple_selector;
-    };
 
-    auto parse_complex_selector = [&]() -> Optional<Selector::CompoundSelector> {
+        } else {
-        auto combinator = Selector::Combinator::Descendant;
+            dbgln_if(CSS_PARSER_DEBUG, "Unexpected Block in pseudo-class name, expected a function or identifier. '{}'", pseudo_class_token.to_debug_string());
-
+            return SelectorParsingResult::SyntaxError;
-        tokens.skip_whitespace();
-
-        auto& current_value = tokens.peek_token();
-        if (current_value.is(Token::Type::Delim)) {
-            auto delim = ((Token)current_value).delim();
-            if (delim == ">") {
-                combinator = Selector::Combinator::ImmediateChild;
-                tokens.next_token();
-            } else if (delim == "+") {
-                combinator = Selector::Combinator::NextSibling;
-                tokens.next_token();
-            } else if (delim == "~") {
-                combinator = Selector::Combinator::SubsequentSibling;
-                tokens.next_token();
-            } else if (delim == "|") {
-                tokens.next_token();
-
-                auto& next = tokens.peek_token();
-                if (next.is(Token::Type::EndOfFile))
-                    return {};
-
-                if (next.is(Token::Type::Delim) && next.token().delim() == "|") {
-                    combinator = Selector::Combinator::Column;
-                    tokens.next_token();
-                }
         }
     }
 
-        tokens.skip_whitespace();
+    dbgln_if(CSS_PARSER_DEBUG, "!!! Invalid simple selector!");
-
+    return SelectorParsingResult::SyntaxError;
-        Vector<Selector::SimpleSelector> simple_selectors;
-
-        for (;;) {
-            auto& current_value = tokens.peek_token();
-            if (current_value.is(Token::Type::EndOfFile) || current_value.is(Token::Type::Whitespace))
-                break;
-
-            auto component = parse_simple_selector();
-            if (!component.has_value())
-                break;
-
-            simple_selectors.append(component.value());
-        }
-
-        if (simple_selectors.is_empty())
-            return {};
-
-        return Selector::CompoundSelector { combinator, move(simple_selectors) };
-    };
-
-    for (;;) {
-        auto& current_value = tokens.peek_token();
-        if (current_value.is(Token::Type::EndOfFile))
-            break;
-
-        auto complex = parse_complex_selector();
-        if (complex.has_value())
-            selectors.append(complex.value());
-    }
-
-    if (selectors.is_empty())
-        return {};
-
-    if (!is_relative)
-        selectors.first().combinator = Selector::Combinator::None;
-
-    return Selector::create(move(selectors));
 }
 
 NonnullRefPtrVector<StyleRule> Parser::consume_a_list_of_rules(bool top_level)
@@ -1179,11 +1251,19 @@ RefPtr<CSSRule> Parser::convert_to_rule(NonnullRefPtr<StyleRule> rule)
     } else {
         auto prelude_stream = TokenStream(rule->m_prelude);
         auto selectors = parse_a_selector(prelude_stream);
+        if (!selectors.has_value() || selectors.value().is_empty()) {
+            dbgln("CSSParser: style rule selectors invalid; discarding.");
+            prelude_stream.dump_all_tokens();
+            return {};
+        }
+
         auto declaration = convert_to_declaration(*rule->m_block);
-        if (declaration && !selectors.is_empty())
+        if (!declaration) {
-            return CSSStyleRule::create(move(selectors), move(*declaration));
+            dbgln("CSSParser: style rule declaration invalid; discarding.");
-        else
+            return {};
-            dbgln("Discarding invalid/unsupported style rule: '{}'", rule->to_string());
+        }
+
+        return CSSStyleRule::create(move(selectors.value()), move(*declaration));
     }
 
     return {};
@@ -1631,8 +1711,6 @@ RefPtr<StyleValue> Parser::parse_image_value(ParsingContext const& context, Styl
 
 RefPtr<StyleValue> Parser::parse_css_value(PropertyID property_id, TokenStream<StyleComponentValueRule>& tokens)
 {
-    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_css_value");
-
     Vector<StyleComponentValueRule> component_values;
 
     while (tokens.has_next_token()) {
@@ -1660,7 +1738,6 @@ RefPtr<StyleValue> Parser::parse_css_value(PropertyID property_id, TokenStream<S
 
 RefPtr<StyleValue> Parser::parse_css_value(ParsingContext const& context, PropertyID property_id, StyleComponentValueRule const& component_value)
 {
-    dbgln_if(CSS_PARSER_DEBUG, "Parser::parse_css_value '{}'", component_value.to_debug_string());
     // FIXME: Figure out if we still need takes_integer_value, and if so, move this information
     // into Properties.json.
     auto takes_integer_value = [](PropertyID property_id) -> bool {

Userland/Libraries/LibWeb/CSS/Parser/Parser.h

@@ -10,6 +10,7 @@
 #include <AK/NonnullOwnPtrVector.h>
 #include <AK/NonnullRefPtrVector.h>
 #include <AK/RefPtr.h>
+#include <AK/Result.h>
 #include <AK/Vector.h>
 #include <LibWeb/CSS/Parser/DeclarationOrAtRule.h>
 #include <LibWeb/CSS/Parser/StyleBlockRule.h>
@@ -111,20 +112,17 @@ public:
     template<typename T>
     Vector<Vector<StyleComponentValueRule>> parse_as_comma_separated_list_of_component_values(TokenStream<T>&);
 
-    template<typename T>
-    RefPtr<Selector> parse_single_selector(TokenStream<T>&, bool is_relative = false);
-
     Optional<Selector::SimpleSelector::NthChildPattern> parse_nth_child_pattern(TokenStream<StyleComponentValueRule>&);
 
     // FIXME: https://www.w3.org/TR/selectors-4/
     // Contrary to the name, these parse a comma-separated list of selectors, according to the spec.
-    NonnullRefPtrVector<Selector> parse_a_selector();
+    Optional<SelectorList> parse_a_selector();
     template<typename T>
-    NonnullRefPtrVector<Selector> parse_a_selector(TokenStream<T>&);
+    Optional<SelectorList> parse_a_selector(TokenStream<T>&);
 
-    NonnullRefPtrVector<Selector> parse_a_relative_selector();
+    Optional<SelectorList> parse_a_relative_selector();
     template<typename T>
-    NonnullRefPtrVector<Selector> parse_a_relative_selector(TokenStream<T>&);
+    Optional<SelectorList> parse_a_relative_selector(TokenStream<T>&);
 
     RefPtr<StyleValue> parse_css_value(PropertyID, TokenStream<StyleComponentValueRule>&);
     static RefPtr<StyleValue> parse_css_value(ParsingContext const&, PropertyID, StyleComponentValueRule const&);
@@ -177,6 +175,21 @@ private:
     static RefPtr<StyleValue> parse_string_value(ParsingContext const&, StyleComponentValueRule const&);
     static RefPtr<StyleValue> parse_image_value(ParsingContext const&, StyleComponentValueRule const&);
 
+    template<typename T>
+    Optional<SelectorList> parse_a_selector_list(TokenStream<T>&);
+    template<typename T>
+    Optional<SelectorList> parse_a_relative_selector_list(TokenStream<T>&);
+
+    enum class SelectorParsingResult {
+        Done,
+        SyntaxError,
+    };
+
+    RefPtr<Selector> parse_complex_selector(TokenStream<StyleComponentValueRule>&, bool allow_starting_combinator);
+    Result<Selector::CompoundSelector, SelectorParsingResult> parse_compound_selector(TokenStream<StyleComponentValueRule>&);
+    Optional<Selector::Combinator> parse_selector_combinator(TokenStream<StyleComponentValueRule>&);
+    Result<Selector::SimpleSelector, SelectorParsingResult> parse_simple_selector(TokenStream<StyleComponentValueRule>&);
+
     ParsingContext m_context;
 
     Tokenizer m_tokenizer;

Userland/Libraries/LibWeb/CSS/Parser/Token.h

@@ -88,6 +88,17 @@ public:
         return m_value.string_view();
     }
 
+    HashType hash_type() const
+    {
+        VERIFY(m_type == Type::Hash);
+        return m_hash_type;
+    }
+    StringView hash_value() const
+    {
+        VERIFY(m_type == Type::Hash);
+        return m_value.string_view();
+    }
+
     bool is(NumberType number_type) const { return is(Token::Type::Number) && m_number_type == number_type; }
 
     int integer() const

Userland/Libraries/LibWeb/CSS/Selector.h

@@ -34,8 +34,8 @@ public:
         Type type { Type::Invalid };
 
         struct NthChildPattern {
-            int step_size = 0;
+            int step_size { 0 };
-            int offset = 0;
+            int offset = { 0 };
 
             static NthChildPattern parse(StringView const& args);
         };
@@ -70,7 +70,7 @@ public:
 
             SelectorList not_selector {};
         };
-        PseudoClass pseudo_class;
+        PseudoClass pseudo_class {};
 
         enum class PseudoElement {
             None,
@@ -81,7 +81,7 @@ public:
         };
         PseudoElement pseudo_element { PseudoElement::None };
 
-        FlyString value;
+        FlyString value {};
 
         struct Attribute {
             enum class MatchType {
@@ -95,10 +95,10 @@ public:
                 EndsWithString,    // [att$=val]
             };
             MatchType match_type { MatchType::None };
-            FlyString name;
+            FlyString name {};
-            String value;
+            String value {};
         };
-        Attribute attribute;
+        Attribute attribute {};
     };
 
     enum class Combinator {