style: Allow :is() and :where() to have better error recovery.

Adjust is-where-parsing.html to work with both the new and old behavior,
and add a test for the new behavior.

Depends on D90049

Differential Revision: https://phabricator.services.mozilla.com/D90050

components/style/gecko/selector_parser.rs

@@ -14,7 +14,7 @@ use crate::string_cache::{Atom, Namespace, WeakAtom, WeakNamespace};
 use crate::values::serialize_atom_identifier;
 use cssparser::{BasicParseError, BasicParseErrorKind, Parser};
 use cssparser::{CowRcStr, SourceLocation, ToCss, Token};
-use selectors::parser::SelectorParseErrorKind;
+use selectors::parser::{SelectorParseErrorKind, ParseErrorRecovery};
 use selectors::parser::{self as selector_parser, Selector};
 use selectors::visitor::SelectorVisitor;
 use selectors::SelectorList;
@@ -341,6 +341,15 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
         true
     }
 
+    #[inline]
+    fn is_and_where_error_recovery(&self) -> ParseErrorRecovery {
+        if static_prefs::pref!("layout.css.is-and-where-better-error-recovery.enabled") {
+            ParseErrorRecovery::IgnoreInvalidSelector
+        } else {
+            ParseErrorRecovery::DiscardList
+        }
+    }
+
     #[inline]
     fn parse_part(&self) -> bool {
         true