Auto merge of #19429 - upsuper:sharing-quirks-id, r=emilio

Don't try to share style on quirks mode whenever two elements have different id

This fixes [bug 1420946](https://bugzilla.mozilla.org/show_bug.cgi?id=1420946).

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components/style/stylist.rs

@@ -26,7 +26,7 @@ use rule_tree::{CascadeLevel, RuleTree, StrongRuleNode, StyleSource};
 use selector_map::{PrecomputedHashMap, SelectorMap, SelectorMapEntry};
 use selector_parser::{SelectorImpl, PerPseudoElementMap, PseudoElement};
 use selectors::NthIndexCache;
-use selectors::attr::NamespaceConstraint;
+use selectors::attr::{CaseSensitivity, NamespaceConstraint};
 use selectors::bloom::{BloomFilter, NonCountingBloomFilter};
 use selectors::matching::{ElementSelectorFlags, matches_selector, MatchingContext, MatchingMode};
 use selectors::matching::VisitedHandlingMode;
@@ -1405,6 +1405,13 @@ impl Stylist {
     where
         E: TElement,
     {
+        // If id needs to be compared case-insensitively, the logic below
+        // wouldn't work. Just conservatively assume it may have such rules.
+        match self.quirks_mode().classes_and_ids_case_sensitivity() {
+            CaseSensitivity::AsciiCaseInsensitive => return true,
+            CaseSensitivity::CaseSensitive => {}
+        }
+
         let hash = id.get_hash();
         for (data, _) in self.cascade_data.iter_origins() {
             if data.mapped_ids.might_contain_hash(hash) {