diff --git a/src/interpreter/style.rs b/src/interpreter/style.rs
index cfb70d6..5b033ff 100644
--- a/src/interpreter/style.rs
+++ b/src/interpreter/style.rs
@@ -461,6 +461,7 @@ impl StyleRule {
 pub struct StyleSheet {
     rules: Vec<StyleRule>,
     index: Option<StyleIndex>,
+    epoch: u64,
 }
 
 impl Default for StyleSheet {
@@ -468,6 +469,7 @@ impl Default for StyleSheet {
         Self {
             rules: Vec::new(),
             index: None,
+            epoch: 0,
         }
     }
 }
@@ -481,11 +483,13 @@ impl StyleSheet {
         for part in split_selectors(&selector) {
             self.rules.push(StyleRule::build(part, properties.clone()));
         }
+        self.index = None;
     }
 
     pub fn build_index(&mut self) {
+        self.epoch += 1;
         let mut index = StyleIndex::new();
-        index.epoch += 1;
+        index.epoch = self.epoch;
 
         for (i, rule) in self.rules.iter().enumerate() {
             let specificity = rule.selector.specificity();
@@ -1205,3 +1209,153 @@ pub fn parse_size(s: &str) -> Option<f32> {
         .parse::<f32>()
         .ok()
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn make_style_sheet(rules: &[(&str, &[(&str, &str)])]) -> StyleSheet {
+        let mut ss = StyleSheet::new();
+        for (selector, props) in rules {
+            let mut map = HashMap::new();
+            for (k, v) in *props {
+                map.insert(k.to_string(), v.to_string());
+            }
+            ss.add_rule(selector.to_string(), map);
+        }
+        ss.build_index();
+        ss
+    }
+
+    #[test]
+    fn test_style_index_basic() {
+        let ss = make_style_sheet(&[
+            ("Button", &[("color", "red")]),
+            ("Button.primary", &[("color", "blue")]),
+            ("#submit", &[("font-size", "20")]),
+            ("Label", &[("color", "green")]),
+        ]);
+
+        let attrs = HashMap::new();
+        let structural = StructuralContext::default();
+
+        // Button class="" should match "Button" rule
+        let matched = ss.matching_rules("Button", None, &[], &[], &structural, &[], &[], &attrs);
+        assert_eq!(matched.len(), 1, "Button should match 1 rule");
+        assert_eq!(matched[0].get("color").map(|s| s.as_str()), Some("red"));
+
+        // Button class="primary" should match both "Button" and "Button.primary"
+        let matched = ss.matching_rules("Button", None, &["primary"], &[], &structural, &[], &[], &attrs);
+        assert_eq!(matched.len(), 2, "Button.primary should match 2 rules");
+        // The more specific rule comes first (or last depending on order)
+        let colors: Vec<&str> = matched.iter().filter_map(|m| m.get("color").map(|s| s.as_str())).collect();
+        assert!(colors.contains(&"red"));
+        assert!(colors.contains(&"blue"));
+
+        // Button id="submit" should match "Button" and "#submit"
+        let matched = ss.matching_rules("Button", Some("submit"), &[], &[], &structural, &[], &[], &attrs);
+        assert_eq!(matched.len(), 2, "Button#submit should match 2 rules");
+        assert!(matched.iter().any(|m| m.contains_key("color")));
+        assert!(matched.iter().any(|m| m.contains_key("font-size")));
+
+        // Label should match "Label"
+        let matched = ss.matching_rules("Label", None, &[], &[], &structural, &[], &[], &attrs);
+        assert_eq!(matched.len(), 1);
+        assert_eq!(matched[0].get("color").map(|s| s.as_str()), Some("green"));
+    }
+
+    #[test]
+    fn test_style_index_empty() {
+        let ss = make_style_sheet(&[]);
+        let attrs = HashMap::new();
+        let structural = StructuralContext::default();
+
+        let matched = ss.matching_rules("Button", None, &[], &[], &structural, &[], &[], &attrs);
+        assert!(matched.is_empty());
+    }
+
+    #[test]
+    fn test_style_index_pseudo() {
+        let ss = make_style_sheet(&[
+            ("Button:hover", &[("color", "red")]),
+            ("Button", &[("color", "blue")]),
+        ]);
+
+        let attrs = HashMap::new();
+        let structural = StructuralContext::default();
+
+        // No pseudo
+        let matched = ss.matching_rules("Button", None, &[], &[], &structural, &[], &[], &attrs);
+        assert_eq!(matched.len(), 1);
+        assert_eq!(matched[0].get("color").map(|s| s.as_str()), Some("blue"));
+
+        // With hover pseudo
+        let matched = ss.matching_rules("Button", None, &[], &["hover"], &structural, &[], &[], &attrs);
+        assert_eq!(matched.len(), 2, "Button:hover should match 2 rules with pseudo");
+    }
+
+    #[test]
+    fn test_style_index_complex() {
+        let ss = make_style_sheet(&[
+            ("Panel > Button", &[("color", "red")]),
+            ("Panel Button", &[("font-size", "16")]),
+        ]);
+
+        let attrs = HashMap::new();
+        let structural = StructuralContext::default();
+
+        // Button with Panel ancestor
+        let ancestors = [AncestorInfo::new("Panel", vec![])];
+        let matched = ss.matching_rules("Button", None, &[], &[], &structural, &ancestors, &[], &attrs);
+        assert_eq!(matched.len(), 2, "Both complex rules should match");
+
+        // Button without Panel ancestor
+        let matched = ss.matching_rules("Button", None, &[], &[], &structural, &[], &[], &attrs);
+        assert_eq!(matched.len(), 0, "No matching without Panel ancestor");
+    }
+
+    #[test]
+    fn test_style_index_off_by_one_same_as_fallback() {
+        // Verify index gives same results as fallback for various inputs
+        let ss = make_style_sheet(&[
+            ("*", &[("margin", "0")]),
+            ("Button", &[("padding", "10")]),
+            ("Button.primary#submit", &[("color", "red")]),
+            ("Label:hover", &[("color", "blue")]),
+            (".highlight", &[("background", "yellow")]),
+        ]);
+
+        let attrs: HashMap<String, String> = HashMap::new();
+        let structural = StructuralContext::default();
+
+        // Verify index matches expected behavior
+        let matched = ss.matching_rules("Button", None, &[], &[], &structural, &[], &[], &attrs);
+        assert!(!matched.is_empty(), "Button should match universal rule");
+        assert!(matched.iter().any(|m| m.get("margin").map(|s| s.as_str()) == Some("0")), "Should include universal margin");
+        assert!(matched.iter().any(|m| m.get("padding").map(|s| s.as_str()) == Some("10")), "Should include Button padding");
+
+        let matched = ss.matching_rules("Button", Some("submit"), &["primary"], &[], &structural, &[], &[], &attrs);
+        assert!(matched.iter().any(|m| m.contains_key("color")), "#submit.primary should match color rule");
+
+        let matched = ss.matching_rules("Label", None, &["highlight"], &[], &structural, &[], &[], &attrs);
+        assert!(matched.iter().any(|m| m.contains_key("background")), "highlight class should match");
+
+        let matched = ss.matching_rules("Panel", None, &[], &[], &structural, &[], &[], &attrs);
+        assert!(matched.iter().any(|m| m.contains_key("margin")), "Panel should match universal rule");
+    }
+
+    #[test]
+    fn test_style_index_epoch_increases() {
+        let mut ss = make_style_sheet(&[("Button", &[("color", "red")])]);
+        let epoch1 = ss.index.as_ref().unwrap().epoch;
+        ss.add_rule("Label".to_string(), {
+            let mut m = HashMap::new();
+            m.insert("color".to_string(), "blue".to_string());
+            m
+        });
+        assert!(ss.index.is_none(), "add_rule should invalidate index");
+        ss.build_index();
+        let epoch2 = ss.index.as_ref().unwrap().epoch;
+        assert!(epoch2 > epoch1, "build_index should increase epoch");
+    }
+}