diff --git a/src/interpreter/mod.rs b/src/interpreter/mod.rs
index 3bb2ed3..1232186 100644
--- a/src/interpreter/mod.rs
+++ b/src/interpreter/mod.rs
@@ -48,6 +48,8 @@ impl Interpreter {
         //let rhei_ctx = RheiContext::new(&rhei_scripts);
         //rhei_ctx.initialize(&mut variables);
 
+        stylesheet.build_index();
+
         Ok(Document { roots, components, variables, rhei_scripts, stylesheet, interner: Interner::new(), tracker })
     }
 
diff --git a/src/interpreter/style.rs b/src/interpreter/style.rs
index f8a41c5..9ce1369 100644
--- a/src/interpreter/style.rs
+++ b/src/interpreter/style.rs
@@ -457,9 +457,19 @@ impl StyleRule {
 }
 
 
-#[derive(Debug, Clone, Default)]
+#[derive(Debug, Clone)]
 pub struct StyleSheet {
     rules: Vec<StyleRule>,
+    index: StyleIndex,
+}
+
+impl Default for StyleSheet {
+    fn default() -> Self {
+        Self {
+            rules: Vec::new(),
+            index: StyleIndex::new(Vec::new()),
+        }
+    }
 }
 
 impl StyleSheet {
@@ -473,6 +483,11 @@ impl StyleSheet {
         }
     }
 
+    /// Rebuild the style index (call after all rules are added).
+    pub fn build_index(&mut self) {
+        self.index = StyleIndex::new(std::mem::take(&mut self.rules));
+    }
+
     pub fn matching_rules<'a>(
         &'a self,
         type_name: &str,
@@ -484,24 +499,10 @@ impl StyleSheet {
         preceding_siblings: &[AncestorInfo],
         el_attributes: &HashMap<String, String>,
     ) -> Vec<&'a HashMap<String, String>> {
-        let mut matched: Vec<(usize, &StyleRule)> = self
-            .rules
-            .iter()
-            .enumerate()
-            .filter(|(_, rule)| {
-                rule.selector
-                    .matches(type_name, el_id, el_classes, active_pseudo, structural, ancestors, preceding_siblings, el_attributes)
-            })
-            .collect();
-
-        matched.sort_by(|(i, a), (j, b)| {
-            a.selector
-                .specificity()
-                .cmp(&b.selector.specificity())
-                .then_with(|| i.cmp(j))
-        });
-
-        matched.into_iter().map(|(_, rule)| &rule.properties).collect()
+        self.index.query(
+            type_name, el_id, el_classes, active_pseudo,
+            structural, ancestors, preceding_siblings, el_attributes,
+        )
     }
 
     pub fn matching_pseudo_rules(
@@ -516,14 +517,12 @@ impl StyleSheet {
         el_attributes: &HashMap<String, String>,
     ) -> HashMap<String, String> {
         let mut props = HashMap::new();
-        for rule in &self.rules {
-            if !rule.selector.has_pseudo_class(pseudo) {
-                continue;
-            }
-            if !rule.selector.matches(type_name, el_id, el_classes, &[pseudo], structural, ancestors, preceding_siblings, el_attributes) {
-                continue;
-            }
-            for (k, v) in &rule.properties {
+        let all_sheets = self.matching_rules(
+            type_name, el_id, el_classes, &[pseudo],
+            structural, ancestors, preceding_siblings, el_attributes,
+        );
+        for sheet in &all_sheets {
+            for (k, v) in sheet.iter() {
                 props.insert(k.clone(), v.clone());
             }
         }
@@ -531,7 +530,212 @@ impl StyleSheet {
     }
 
     pub fn is_empty(&self) -> bool {
-        self.rules.is_empty()
+        self.index.rules.is_empty()
+    }
+}
+
+type RuleId = usize;
+
+/// Pre-built index for O(1) rule candidate lookup instead of O(N) scan.
+#[derive(Debug, Clone)]
+pub struct StyleIndex {
+    /// Rules keyed by tag name (e.g. "Button", "Panel")
+    by_tag: HashMap<String, Vec<RuleId>>,
+    /// Rules keyed by class name
+    by_class: HashMap<String, Vec<RuleId>>,
+    /// Rules keyed by id
+    by_id: HashMap<String, Vec<RuleId>>,
+    /// Rules keyed by (tag, class) pair
+    by_tag_class: HashMap<(String, String), Vec<RuleId>>,
+    /// Rules keyed by (tag, id) pair
+    by_tag_id: HashMap<(String, String), Vec<RuleId>>,
+    /// Complex selectors that can't be indexed (combinators, pseudo-classes, etc.)
+    complex_rules: Vec<(ComplexSelector, RuleId)>,
+    /// All rules, sorted by specificity once at build time
+    rules: Vec<StyleRule>,
+    /// Epoch counter: incremented on rebuild, used for cache invalidation
+    epoch: u64,
+}
+
+impl StyleIndex {
+    pub fn new(rules: Vec<StyleRule>) -> Self {
+        let mut idx = Self {
+            by_tag: HashMap::new(),
+            by_class: HashMap::new(),
+            by_id: HashMap::new(),
+            by_tag_class: HashMap::new(),
+            by_tag_id: HashMap::new(),
+            complex_rules: Vec::new(),
+            rules: Vec::new(),
+            epoch: 0,
+        };
+
+        // Sort rules by specificity once
+        let mut sorted_rules: Vec<(usize, StyleRule)> = rules.into_iter().enumerate().collect();
+        sorted_rules.sort_by(|(i, a), (j, b)| {
+            a.selector
+                .specificity()
+                .cmp(&b.selector.specificity())
+                .then_with(|| i.cmp(j))
+        });
+
+        for (new_id, (_, rule)) in sorted_rules.into_iter().enumerate() {
+            let rid = idx.rules.len();
+            idx.rules.push(rule);
+
+            let simple = idx.rules[rid].selector.as_simple().cloned();
+
+            if let Some(compound) = simple {
+                let has_pseudo = !compound.pseudo_classes.is_empty();
+                let has_complex_combinator = false; // simple selector = no combinators
+
+                if has_pseudo || has_complex_combinator {
+                    idx.complex_rules.push((idx.rules[rid].selector.clone(), rid));
+                } else {
+                    // Index by tag
+                    if let Some(ref tag) = compound.tag {
+                        idx.by_tag.entry(tag.clone()).or_default().push(rid);
+                        if let Some(ref id) = compound.id {
+                            idx.by_tag_id.entry((tag.clone(), id.clone())).or_default().push(rid);
+                        }
+                        for cls in &compound.classes {
+                            idx.by_tag_class.entry((tag.clone(), cls.clone())).or_default().push(rid);
+                        }
+                    }
+                    // Index by class
+                    for cls in &compound.classes {
+                        idx.by_class.entry(cls.clone()).or_default().push(rid);
+                    }
+                    // Index by id
+                    if let Some(ref id) = compound.id {
+                        idx.by_id.entry(id.clone()).or_default().push(rid);
+                    }
+                }
+            } else {
+                // Complex selector (combinators, multiple compounds)
+                idx.complex_rules.push((idx.rules[rid].selector.clone(), rid));
+            }
+        }
+
+        idx
+    }
+
+    pub fn epoch(&self) -> u64 {
+        self.epoch
+    }
+
+    pub fn query<'a>(
+        &'a self,
+        type_name: &str,
+        el_id: Option<&str>,
+        el_classes: &[&str],
+        active_pseudo: &[&str],
+        structural: &StructuralContext,
+        ancestors: &[AncestorInfo],
+        preceding_siblings: &[AncestorInfo],
+        el_attributes: &HashMap<String, String>,
+    ) -> Vec<&'a HashMap<String, String>> {
+        // Collect candidates from indexes
+        let mut candidates: Vec<RuleId> = Vec::new();
+
+        // Start with tag index
+        if let Some(tag_rules) = self.by_tag.get(type_name) {
+            // If we have tag+id, prefer that for more precise filtering
+            if let Some(id) = el_id {
+                if let Some(tag_id_rules) = self.by_tag_id.get(&(type_name.to_string(), id.to_string())) {
+                    candidates = tag_id_rules.clone();
+                } else {
+                    candidates = tag_rules.clone();
+                }
+            } else {
+                candidates = tag_rules.clone();
+            }
+
+            // Intersect with class if applicable
+            if !el_classes.is_empty() {
+                let first_class = el_classes[0];
+                if let Some(tag_class_rules) = self.by_tag_class.get(&(type_name.to_string(), first_class.to_string())) {
+                    candidates.retain(|r| tag_class_rules.contains(r));
+                } else {
+                    // No rules for tag+class, clear non-matching
+                    candidates.retain(|r| {
+                        el_classes.iter().any(|cls| {
+                            self.by_class.get(*cls).map_or(false, |cr| cr.contains(r))
+                        })
+                    });
+                }
+            }
+
+            // Also check additional class-only rules
+            for cls in el_classes.iter().skip(if el_id.is_some() { 0 } else { 1 }) {
+                if let Some(cls_rules) = self.by_class.get(*cls) {
+                    for &r in cls_rules {
+                        if !candidates.contains(&r) {
+                            candidates.push(r);
+                        }
+                    }
+                }
+            }
+
+            // Also check id-only rules
+            if let Some(id) = el_id {
+                if let Some(id_rules) = self.by_id.get(id) {
+                    for &r in id_rules {
+                        if !candidates.contains(&r) {
+                            candidates.push(r);
+                        }
+                    }
+                }
+            }
+        } else {
+            // No tag match — check class and id indexes
+            for cls in el_classes {
+                if let Some(cls_rules) = self.by_class.get(*cls) {
+                    candidates.extend(cls_rules);
+                }
+            }
+            if let Some(id) = el_id {
+                if let Some(id_rules) = self.by_id.get(id) {
+                    candidates.extend(id_rules);
+                }
+            }
+        }
+
+        // De-duplicate while preserving order
+        let mut seen = std::collections::HashSet::new();
+        candidates.retain(|r| seen.insert(*r));
+
+        // Test candidates against full selector
+        let mut matched: Vec<&StyleRule> = Vec::new();
+        for &rid in &candidates {
+            let rule = &self.rules[rid];
+            if rule.selector.matches(
+                type_name, el_id, el_classes, active_pseudo,
+                structural, ancestors, preceding_siblings, el_attributes,
+            ) {
+                matched.push(rule);
+            }
+        }
+
+        // Test complex rules
+        for (sel, rid) in &self.complex_rules {
+            if sel.matches(
+                type_name, el_id, el_classes, active_pseudo,
+                structural, ancestors, preceding_siblings, el_attributes,
+            ) {
+                matched.push(&self.rules[*rid]);
+            }
+        }
+
+        // Re-sort matched by specificity (candidates are already sorted)
+        // but we only need to sort the matched subset
+        matched.sort_by(|a, b| {
+            a.selector
+                .specificity()
+                .cmp(&b.selector.specificity())
+        });
+
+        matched.into_iter().map(|r| &r.properties).collect()
     }
 }