diff --git a/servo/components/style/gecko/generated/bindings.rs b/servo/components/style/gecko/generated/bindings.rs
index 66b551524156..422b2ddfbedb 100644
--- a/servo/components/style/gecko/generated/bindings.rs
+++ b/servo/components/style/gecko/generated/bindings.rs
@@ -1655,24 +1655,24 @@ extern "C" {
 extern "C" {
     pub fn Servo_StyleSet_AppendStyleSheet(set: RawServoStyleSetBorrowed,
                                            sheet: RawServoStyleSheetBorrowed,
-                                           unique_id: u32);
+                                           unique_id: u64);
 }
 extern "C" {
     pub fn Servo_StyleSet_PrependStyleSheet(set: RawServoStyleSetBorrowed,
                                             sheet: RawServoStyleSheetBorrowed,
-                                            unique_id: u32);
+                                            unique_id: u64);
 }
 extern "C" {
     pub fn Servo_StyleSet_RemoveStyleSheet(set: RawServoStyleSetBorrowed,
-                                           unique_id: u32);
+                                           unique_id: u64);
 }
 extern "C" {
     pub fn Servo_StyleSet_InsertStyleSheetBefore(set:
                                                      RawServoStyleSetBorrowed,
                                                  sheet:
                                                      RawServoStyleSheetBorrowed,
-                                                 unique_id: u32,
-                                                 before_unique_id: u32);
+                                                 unique_id: u64,
+                                                 before_unique_id: u64);
 }
 extern "C" {
     pub fn Servo_StyleSet_FlushStyleSheets(set: RawServoStyleSetBorrowed);
diff --git a/servo/components/style/stylesheet_set.rs b/servo/components/style/stylesheet_set.rs
index da4e75e5ecaa..c21084892a7f 100644
--- a/servo/components/style/stylesheet_set.rs
+++ b/servo/components/style/stylesheet_set.rs
@@ -11,7 +11,7 @@ use stylesheets::Stylesheet;
 /// Entry for a StylesheetSet. We don't bother creating a constructor, because
 /// there's no sensible defaults for the member variables.
 pub struct StylesheetSetEntry {
-    unique_id: u32,
+    unique_id: u64,
     sheet: Arc<Stylesheet>,
 }
 
@@ -58,13 +58,13 @@ impl StylesheetSet {
         self.author_style_disabled
     }
 
-    fn remove_stylesheet_if_present(&mut self, unique_id: u32) {
+    fn remove_stylesheet_if_present(&mut self, unique_id: u64) {
         self.entries.retain(|x| x.unique_id != unique_id);
     }
 
     /// Appends a new stylesheet to the current set.
     pub fn append_stylesheet(&mut self, sheet: &Arc<Stylesheet>,
-                             unique_id: u32) {
+                             unique_id: u64) {
         self.remove_stylesheet_if_present(unique_id);
         self.entries.push(StylesheetSetEntry {
             unique_id: unique_id,
@@ -75,7 +75,7 @@ impl StylesheetSet {
 
     /// Prepend a new stylesheet to the current set.
     pub fn prepend_stylesheet(&mut self, sheet: &Arc<Stylesheet>,
-                              unique_id: u32) {
+                              unique_id: u64) {
         self.remove_stylesheet_if_present(unique_id);
         self.entries.insert(0, StylesheetSetEntry {
             unique_id: unique_id,
@@ -87,8 +87,8 @@ impl StylesheetSet {
     /// Insert a given stylesheet before another stylesheet in the document.
     pub fn insert_stylesheet_before(&mut self,
                                     sheet: &Arc<Stylesheet>,
-                                    unique_id: u32,
-                                    before_unique_id: u32) {
+                                    unique_id: u64,
+                                    before_unique_id: u64) {
         self.remove_stylesheet_if_present(unique_id);
         let index = self.entries.iter().position(|x| {
             x.unique_id == before_unique_id
@@ -101,7 +101,7 @@ impl StylesheetSet {
     }
 
     /// Remove a given stylesheet from the set.
-    pub fn remove_stylesheet(&mut self, unique_id: u32) {
+    pub fn remove_stylesheet(&mut self, unique_id: u64) {
         self.remove_stylesheet_if_present(unique_id);
         self.dirty = true;
     }
diff --git a/servo/ports/geckolib/glue.rs b/servo/ports/geckolib/glue.rs
index 9bf7babc6375..acf5edd0800d 100644
--- a/servo/ports/geckolib/glue.rs
+++ b/servo/ports/geckolib/glue.rs
@@ -617,7 +617,7 @@ pub extern "C" fn Servo_StyleSheet_ClearAndUpdate(stylesheet: RawServoStyleSheet
 #[no_mangle]
 pub extern "C" fn Servo_StyleSet_AppendStyleSheet(raw_data: RawServoStyleSetBorrowed,
                                                   raw_sheet: RawServoStyleSheetBorrowed,
-                                                  unique_id: u32) {
+                                                  unique_id: u64) {
     let mut data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
     let sheet = HasArcFFI::as_arc(&raw_sheet);
     data.stylesheets.append_stylesheet(sheet, unique_id);
@@ -627,7 +627,7 @@ pub extern "C" fn Servo_StyleSet_AppendStyleSheet(raw_data: RawServoStyleSetBorr
 #[no_mangle]
 pub extern "C" fn Servo_StyleSet_PrependStyleSheet(raw_data: RawServoStyleSetBorrowed,
                                                    raw_sheet: RawServoStyleSheetBorrowed,
-                                                   unique_id: u32) {
+                                                   unique_id: u64) {
     let mut data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
     let sheet = HasArcFFI::as_arc(&raw_sheet);
     data.stylesheets.prepend_stylesheet(sheet, unique_id);
@@ -637,8 +637,8 @@ pub extern "C" fn Servo_StyleSet_PrependStyleSheet(raw_data: RawServoStyleSetBor
 #[no_mangle]
 pub extern "C" fn Servo_StyleSet_InsertStyleSheetBefore(raw_data: RawServoStyleSetBorrowed,
                                                         raw_sheet: RawServoStyleSheetBorrowed,
-                                                        unique_id: u32,
-                                                        before_unique_id: u32) {
+                                                        unique_id: u64,
+                                                        before_unique_id: u64) {
     let mut data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
     let sheet = HasArcFFI::as_arc(&raw_sheet);
     data.stylesheets.insert_stylesheet_before(sheet, unique_id, before_unique_id);
@@ -647,7 +647,7 @@ pub extern "C" fn Servo_StyleSet_InsertStyleSheetBefore(raw_data: RawServoStyleS
 
 #[no_mangle]
 pub extern "C" fn Servo_StyleSet_RemoveStyleSheet(raw_data: RawServoStyleSetBorrowed,
-                                                  unique_id: u32) {
+                                                  unique_id: u64) {
     let mut data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
     data.stylesheets.remove_stylesheet(unique_id);
     data.clear_stylist();