fix(dom): honor quirks mode for class/id selector case-sensitivity

set_quirks_mode was a no-op and the selector MatchingContext hardcoded
QuirksMode::NoQuirks, so class/id selectors were always case-sensitive even on
quirks-mode (no-doctype) pages where browsers match .foo / #bar
ASCII-case-insensitively. Store the parser-reported quirks mode on DomTree, pass
it into the selector context, and skip the exact-case #id index fast path in
quirks mode so it falls through to case-insensitive matching.

Fixes #499

crates/obscura-dom/src/selector.rs

@@ -580,6 +580,10 @@ impl DomTree {
         // Fast path: a bare "#id" selector resolves through the id index in O(1)
         // instead of scanning every descendant. The index holds the first element
         // in tree order per id, which is exactly what the full scan would return.
+        // In quirks mode `#id` matches ASCII-case-insensitively, but the id
+        // index is keyed on the exact-case id, so skip the fast path and let the
+        // selector engine (below) do the case-insensitive match.
+        if !self.is_quirks() {
             if let Some(id) = simple_id_selector(selector) {
                 match self.get_element_by_id(id) {
                     // querySelector matches strict descendants of root only, so the
@@ -592,13 +596,14 @@ impl DomTree {
                     _ => {}
                 }
             }
+        }
         let selector_list = parse_selector(selector)?;
         let mut caches = selectors::context::SelectorCaches::default();
         let mut context = MatchingContext::new(
             MatchingMode::Normal,
             None,
             &mut caches,
-            QuirksMode::NoQuirks,
+            self.selector_quirks_mode(),
             NeedsSelectorFlags::No,
             MatchingForInvalidation::No,
         );
@@ -619,6 +624,16 @@ impl DomTree {
         Ok(None)
     }
 
+    // Map the document's quirks flag onto the selector crate's QuirksMode. In
+    // quirks mode the crate matches class/id selectors ASCII-case-insensitively.
+    fn selector_quirks_mode(&self) -> QuirksMode {
+        if self.is_quirks() {
+            QuirksMode::Quirks
+        } else {
+            QuirksMode::NoQuirks
+        }
+    }
+
     pub fn query_selector_all_from(&self, root: NodeId, selector: &str) -> Result<Vec<NodeId>, String> {
         let selector_list = parse_selector(selector)?;
         let mut caches = selectors::context::SelectorCaches::default();
@@ -626,7 +641,7 @@ impl DomTree {
             MatchingMode::Normal,
             None,
             &mut caches,
-            QuirksMode::NoQuirks,
+            self.selector_quirks_mode(),
             NeedsSelectorFlags::No,
             MatchingForInvalidation::No,
         );
@@ -729,6 +744,34 @@ mod tests {
         assert!(result.is_none());
     }
 
+    #[test]
+    fn quirks_mode_matches_class_and_id_case_insensitively() {
+        // No doctype => quirks mode; class/id match ASCII case-insensitively.
+        let tree = parse_html(r#"<div class="Foo" id="Bar">x</div>"#);
+        assert!(
+            tree.query_selector(".foo").unwrap().is_some(),
+            ".foo should match class=\"Foo\" in quirks mode"
+        );
+        assert!(
+            tree.query_selector("#bar").unwrap().is_some(),
+            "#bar should match id=\"Bar\" in quirks mode"
+        );
+    }
+
+    #[test]
+    fn standards_mode_matches_class_and_id_case_sensitively() {
+        // With a doctype => no-quirks; class/id remain case-sensitive.
+        let tree = parse_html(r#"<!DOCTYPE html><div class="Foo" id="Bar">x</div>"#);
+        assert!(
+            tree.query_selector(".foo").unwrap().is_none(),
+            ".foo must NOT match class=\"Foo\" in standards mode"
+        );
+        assert!(
+            tree.query_selector("#bar").unwrap().is_none(),
+            "#bar must NOT match id=\"Bar\" in standards mode"
+        );
+    }
+
     #[test]
     fn test_query_selector_complex() {
         let tree = parse_html(

crates/obscura-dom/src/tree.rs

@@ -143,6 +143,9 @@ pub(crate) struct DomTreeInner {
     pub(crate) free_list: Vec<u32>,
     pub(crate) document: NodeId,
     pub(crate) id_index: HashMap<String, NodeId>,
+    // Whether the document was parsed in (full) quirks mode. In quirks mode CSS
+    // class and id selectors match ASCII-case-insensitively.
+    pub(crate) quirks: bool,
 }
 
 impl DomTree {
@@ -162,6 +165,7 @@ impl DomTree {
                 free_list: Vec::new(),
                 document: NodeId(0),
                 id_index: HashMap::new(),
+                quirks: false,
             }),
         }
     }
@@ -170,6 +174,17 @@ impl DomTree {
         self.inner.borrow().document
     }
 
+    /// Record whether the document was parsed in (full) quirks mode.
+    pub fn set_quirks(&self, quirks: bool) {
+        self.inner.borrow_mut().quirks = quirks;
+    }
+
+    /// Whether the document is in (full) quirks mode, in which CSS class and id
+    /// selectors match ASCII-case-insensitively.
+    pub fn is_quirks(&self) -> bool {
+        self.inner.borrow().quirks
+    }
+
     pub(crate) fn borrow_inner(&self) -> std::cell::Ref<'_, DomTreeInner> {
         self.inner.borrow()
     }

crates/obscura-dom/src/tree_sink.rs

@@ -218,7 +218,10 @@ impl TreeSink for DomTree {
         x == y
     }
 
-    fn set_quirks_mode(&self, _mode: QuirksMode) {
+    fn set_quirks_mode(&self, mode: QuirksMode) {
+        // Only full quirks mode makes CSS class/id selectors case-insensitive;
+        // limited-quirks behaves like no-quirks for selector matching.
+        self.set_quirks(mode == QuirksMode::Quirks);
     }
 
     fn is_mathml_annotation_xml_integration_point(&self, target: &NodeId) -> bool {