LibWeb: Parse forgiving selector-lists

`<forgiving-selector-list>` and `<forgiving-relative-selector-list>` are
the same as regular selector-lists, except that an invalid selector
does not make the whole list invalid. The former is used by the `:is()`
pseudo-class.

For example:

```css
/* This entire selector-list is invalid */
.foo, .bar, !?invalid { }

/* This is valid, but the "!?invalid" selector is removed */
:is(.foo, .bar, !?invalid) { }
```

Also as part of this, I've removed the `parse_a_selector(TokenStream)`
and `parse_a_relative_selector(TokenStream)` methods as they don't add
anything useful.

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

@@ -179,71 +179,65 @@ NonnullRefPtr<CSSStyleSheet> Parser::parse_a_stylesheet(TokenStream<T>& tokens)
  return CSSStyleSheet::create(rules);
 }
 
-Optional<SelectorList> Parser::parse_as_selector()
+Optional<SelectorList> Parser::parse_as_selector(SelectorParsingMode parsing_mode)
 {
- auto selector_list = parse_a_selector(m_token_stream);
+ auto selector_list = parse_a_selector_list(m_token_stream, parsing_mode);
  if (!selector_list.is_error())
  return selector_list.release_value();
 
  return {};
 }
 
-template<typename T>
-Result<SelectorList, Parser::ParsingResult> Parser::parse_a_selector(TokenStream<T>& tokens)
-{
- return parse_a_selector_list(tokens);
-}
-
-Optional<SelectorList> Parser::parse_as_relative_selector()
+Optional<SelectorList> Parser::parse_as_relative_selector(SelectorParsingMode parsing_mode)
 {
- auto selector_list = parse_a_relative_selector(m_token_stream);
+ auto selector_list = parse_a_relative_selector_list(m_token_stream, parsing_mode);
  if (!selector_list.is_error())
  return selector_list.release_value();
 
  return {};
 }
 
 template<typename T>
-Result<SelectorList, Parser::ParsingResult> Parser::parse_a_relative_selector(TokenStream<T>& tokens)
-{
- return parse_a_relative_selector_list(tokens);
-}
-
-template<typename T>
-Result<SelectorList, Parser::ParsingResult> Parser::parse_a_selector_list(TokenStream<T>& tokens)
+Result<SelectorList, Parser::ParsingResult> Parser::parse_a_selector_list(TokenStream<T>& tokens, SelectorParsingMode parsing_mode)
 {
  auto comma_separated_lists = parse_a_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_complex_selector(stream, false);
- if (selector.is_error())
+ if (selector.is_error()) {
+ if (parsing_mode == SelectorParsingMode::Forgiving)
+ continue;
  return selector.error();
+ }
  selectors.append(selector.release_value());
  }
 
- if (selectors.is_empty())
+ if (selectors.is_empty() && parsing_mode != SelectorParsingMode::Forgiving)
  return ParsingResult::SyntaxError;
 
  return selectors;
 }
 
 template<typename T>
-Result<SelectorList, Parser::ParsingResult> Parser::parse_a_relative_selector_list(TokenStream<T>& tokens)
+Result<SelectorList, Parser::ParsingResult> Parser::parse_a_relative_selector_list(TokenStream<T>& tokens, SelectorParsingMode parsing_mode)
 {
  auto comma_separated_lists = parse_a_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_complex_selector(stream, true);
- if (selector.is_error())
+ if (selector.is_error()) {
+ if (parsing_mode == SelectorParsingMode::Forgiving)
+ continue;
  return selector.error();
+ }
  selectors.append(selector.release_value());
  }
 
- if (selectors.is_empty())
+ if (selectors.is_empty() && parsing_mode != SelectorParsingMode::Forgiving)
  return ParsingResult::SyntaxError;
 
  return selectors;
@@ -593,7 +587,7 @@ Result<Selector::SimpleSelector, Parser::ParsingResult> Parser::parse_simple_sel
  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);
+ auto not_selector = parse_a_selector_list(function_token_stream);
  if (not_selector.is_error()) {
  dbgln_if(CSS_PARSER_DEBUG, "Invalid selector in :not() clause");
  return ParsingResult::SyntaxError;
@@ -2087,7 +2081,7 @@ RefPtr<CSSRule> Parser::convert_to_rule(NonnullRefPtr<StyleRule> rule)
 
  } else {
  auto prelude_stream = TokenStream(rule->m_prelude);
- auto selectors = parse_a_selector(prelude_stream);
+ auto selectors = parse_a_selector_list(prelude_stream);
 
  if (selectors.is_error()) {
  if (selectors.error() != ParsingResult::IncludesIgnoredVendorPrefix) {

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

@@ -105,9 +105,16 @@ class Parser {
  Vector<StyleComponentValueRule> parse_as_list_of_component_values();
  Vector<Vector<StyleComponentValueRule>> parse_as_comma_separated_list_of_component_values();
 
+ enum class SelectorParsingMode {
+ Standard,
+ // `<forgiving-selector-list>` and `<forgiving-relative-selector-list>`
+ // are handled with this parameter, not as separate functions.
+ // https://drafts.csswg.org/selectors/#forgiving-selector
+ Forgiving
+ };
  // Contrary to the name, these parse a comma-separated list of selectors, according to the spec.
- Optional<SelectorList> parse_as_selector();
- Optional<SelectorList> parse_as_relative_selector();
+ Optional<SelectorList> parse_as_selector(SelectorParsingMode = SelectorParsingMode::Standard);
+ Optional<SelectorList> parse_as_relative_selector(SelectorParsingMode = SelectorParsingMode::Standard);
 
  NonnullRefPtrVector<MediaQuery> parse_as_media_query_list();
  RefPtr<MediaQuery> parse_as_media_query();
@@ -142,13 +149,9 @@ class Parser {
  template<typename T>
  Vector<Vector<StyleComponentValueRule>> parse_a_comma_separated_list_of_component_values(TokenStream<T>&);
  template<typename T>
- Result<SelectorList, ParsingResult> parse_a_selector(TokenStream<T>&);
- template<typename T>
- Result<SelectorList, ParsingResult> parse_a_relative_selector(TokenStream<T>&);
- template<typename T>
- Result<SelectorList, ParsingResult> parse_a_selector_list(TokenStream<T>&);
+ Result<SelectorList, ParsingResult> parse_a_selector_list(TokenStream<T>&, SelectorParsingMode = SelectorParsingMode::Standard);
  template<typename T>
- Result<SelectorList, ParsingResult> parse_a_relative_selector_list(TokenStream<T>&);
+ Result<SelectorList, ParsingResult> parse_a_relative_selector_list(TokenStream<T>&, SelectorParsingMode = SelectorParsingMode::Standard);
  template<typename T>
  NonnullRefPtrVector<MediaQuery> parse_a_media_query_list(TokenStream<T>&);
  template<typename T>