diff --git a/components/script/dom/cssrulelist.rs b/components/script/dom/cssrulelist.rs
index 5975a5eb6d1..1c383403a00 100644
--- a/components/script/dom/cssrulelist.rs
+++ b/components/script/dom/cssrulelist.rs
@@ -25,7 +25,7 @@ pub struct CSSRuleList {
 impl CSSRuleList {
     #[allow(unrooted_must_root)]
     pub fn new_inherited(sheet: &CSSStyleSheet, rules: CssRules) -> CSSRuleList {
-        let dom_rules = rules.0.iter().map(|_| MutNullableHeap::new(None)).collect();
+        let dom_rules = rules.0.read().iter().map(|_| MutNullableHeap::new(None)).collect();
         CSSRuleList {
             reflector_: Reflector::new(),
             sheet: JS::from_ref(sheet),
@@ -49,7 +49,7 @@ impl CSSRuleListMethods for CSSRuleList {
             rule.or_init(|| {
                 CSSRule::new_specific(self.global().as_window(),
                                      &self.sheet,
-                                     self.rules.0[idx as usize].clone())
+                                     self.rules.0.read()[idx as usize].clone())
             })
         })
     }
diff --git a/components/style/selector_matching.rs b/components/style/selector_matching.rs
index b7c0df114d1..c574c097a5a 100644
--- a/components/style/selector_matching.rs
+++ b/components/style/selector_matching.rs
@@ -380,7 +380,7 @@ impl Stylist {
                             return true
                         }
                     }
-                    mq_eval_changed(&rules, before, after)
+                    mq_eval_changed(rules, before, after)
                 }) {
                     return true
                 }
@@ -388,7 +388,7 @@ impl Stylist {
             false
         }
         self.is_device_dirty |= stylesheets.iter().any(|stylesheet| {
-            mq_eval_changed(&stylesheet.rules.0, &self.device, &device)
+            mq_eval_changed(&stylesheet.rules.0.read(), &self.device, &device)
         });
 
         self.device = device;
diff --git a/components/style/stylesheets.rs b/components/style/stylesheets.rs
index 7126ba0ee8c..1538e9e0f25 100644
--- a/components/style/stylesheets.rs
+++ b/components/style/stylesheets.rs
@@ -43,11 +43,11 @@ pub enum Origin {
 }
 
 #[derive(Debug, Clone)]
-pub struct CssRules(pub Arc<Vec<CssRule>>);
+pub struct CssRules(pub Arc<RwLock<Vec<CssRule>>>);
 
 impl From<Vec<CssRule>> for CssRules {
     fn from(other: Vec<CssRule>) -> Self {
-        CssRules(Arc::new(other))
+        CssRules(Arc::new(RwLock::new(other)))
     }
 }
 
@@ -100,7 +100,8 @@ impl CssRule {
             CssRule::Media(ref lock) => {
                 let media_rule = lock.read();
                 let mq = media_rule.media_queries.read();
-                f(&media_rule.rules.0, Some(&mq))
+                let rules = media_rule.rules.0.read();
+                f(&rules, Some(&mq))
             }
         }
     }
@@ -254,7 +255,7 @@ impl Stylesheet {
     /// examined.
     #[inline]
     pub fn effective_rules<F>(&self, device: &Device, mut f: F) where F: FnMut(&CssRule) {
-        effective_rules(&self.rules.0, device, &mut f);
+        effective_rules(&self.rules.0.read(), device, &mut f);
     }
 }
 
diff --git a/ports/geckolib/glue.rs b/ports/geckolib/glue.rs
index cc672475c90..8a1745c16ca 100644
--- a/ports/geckolib/glue.rs
+++ b/ports/geckolib/glue.rs
@@ -255,7 +255,7 @@ pub extern "C" fn Servo_StyleSet_RemoveStyleSheet(raw_data: RawServoStyleSetBorr
 
 #[no_mangle]
 pub extern "C" fn Servo_StyleSheet_HasRules(raw_sheet: RawServoStyleSheetBorrowed) -> bool {
-    !Stylesheet::as_arc(&raw_sheet).rules.0.is_empty()
+    !Stylesheet::as_arc(&raw_sheet).rules.0.read().is_empty()
 }
 
 #[no_mangle]
diff --git a/tests/unit/style/media_queries.rs b/tests/unit/style/media_queries.rs
index 20375317bc2..59628128082 100644
--- a/tests/unit/style/media_queries.rs
+++ b/tests/unit/style/media_queries.rs
@@ -29,7 +29,7 @@ fn test_media_rule<F>(css: &str, callback: F) where F: Fn(&MediaList, &str) {
     let stylesheet = Stylesheet::from_str(css, url, Origin::Author, Box::new(CSSErrorReporterTest),
                                           ParserContextExtraData::default());
     let mut rule_count = 0;
-    media_queries(&stylesheet.rules.0, &mut |mq| {
+    media_queries(&stylesheet.rules.0.read(), &mut |mq| {
         rule_count += 1;
         callback(mq, css);
     });