--- a/servo/components/style/matching.rs
+++ b/servo/components/style/matching.rs
@@ -467,17 +467,17 @@ trait PrivateMatchMethods: TElement {
                 return current;
             }
         }
     }
 
     fn cascade_internal(&self,
                         context: &StyleContext<Self>,
                         primary_style: &ComputedStyle,
-                        pseudo_style: &mut Option<(&PseudoElement, &mut ComputedStyle)>,
+                        pseudo_style: &Option<(&PseudoElement, &mut ComputedStyle)>,
                         booleans: &CascadeBooleans)
                         -> Arc<ComputedValues> {
         let shared_context = context.shared;
         let mut cascade_info = CascadeInfo::new();
         let mut cascade_flags = CascadeFlags::empty();
         if booleans.shareable {
             cascade_flags.insert(SHAREABLE)
         }
@@ -565,17 +565,17 @@ trait PrivateMatchMethods: TElement {
         let mut primary_style = &mut styles.primary;
         let pseudos = &mut styles.pseudos;
         let mut pseudo_style = pseudo.map(|p| (p, pseudos.get_mut(p).unwrap()));
         let mut old_values =
             pseudo_style.as_mut().map_or_else(|| primary_style.values.take(), |p| p.1.values.take());
 
         // Compute the new values.
         let mut new_values = self.cascade_internal(context, primary_style,
-                                                   &mut pseudo_style, &booleans);
+                                                   &pseudo_style, &booleans);
 
         // Handle animations.
         if booleans.animate {
             self.process_animations(context,
                                     &mut old_values,
                                     &mut new_values,
                                     pseudo,
                                     possibly_expired_animations);