--- a/layout/style/CSSStyleSheet.cpp
+++ b/layout/style/CSSStyleSheet.cpp
@@ -350,25 +350,28 @@ CSSStyleSheet::CSSStyleSheet(css::SheetP
   mInner->AddSheet(this);
 }
 
 CSSStyleSheet::CSSStyleSheet(const CSSStyleSheet& aCopy,
                              CSSStyleSheet* aParentToUse,
                              dom::CSSImportRule* aOwnerRuleToUse,
                              nsIDocument* aDocumentToUse,
                              nsINode* aOwningNodeToUse)
-  : StyleSheet(aCopy, aOwnerRuleToUse, aDocumentToUse, aOwningNodeToUse),
-    mInRuleProcessorCache(false),
-    mScopeElement(nullptr),
-    mRuleProcessors(nullptr)
+  : StyleSheet(aCopy,
+               aParentToUse,
+               aOwnerRuleToUse,
+               aDocumentToUse,
+               aOwningNodeToUse)
+  , mInRuleProcessorCache(false)
+  , mScopeElement(nullptr)
+  , mRuleProcessors(nullptr)
 {
-  mParent = aParentToUse;
-
   if (mDirty) { // CSSOM's been there, force full copy now
-    NS_ASSERTION(mInner->mComplete, "Why have rules been accessed on an incomplete sheet?");
+    NS_ASSERTION(mInner->mComplete,
+                 "Why have rules been accessed on an incomplete sheet?");
     // FIXME: handle failure?
     //
     // NOTE: It's important to call this from the subclass, since it could
     // access uninitialized members otherwise.
     EnsureUniqueInner();
   }
 }
 

--- a/layout/style/ServoStyleSheet.cpp
+++ b/layout/style/ServoStyleSheet.cpp
@@ -79,22 +79,25 @@ ServoStyleSheet::ServoStyleSheet(css::Sh
   mInner->AddSheet(this);
 }
 
 ServoStyleSheet::ServoStyleSheet(const ServoStyleSheet& aCopy,
                                  ServoStyleSheet* aParentToUse,
                                  dom::CSSImportRule* aOwnerRuleToUse,
                                  nsIDocument* aDocumentToUse,
                                  nsINode* aOwningNodeToUse)
-  : StyleSheet(aCopy, aOwnerRuleToUse, aDocumentToUse, aOwningNodeToUse)
+  : StyleSheet(aCopy,
+               aParentToUse,
+               aOwnerRuleToUse,
+               aDocumentToUse,
+               aOwningNodeToUse)
 {
-  mParent = aParentToUse;
-
   if (mDirty) { // CSSOM's been there, force full copy now
-    NS_ASSERTION(mInner->mComplete, "Why have rules been accessed on an incomplete sheet?");
+    NS_ASSERTION(mInner->mComplete,
+                 "Why have rules been accessed on an incomplete sheet?");
     // FIXME: handle failure?
     //
     // NOTE: It's important to call this from the subclass, since this could
     // access uninitialized members otherwise.
     EnsureUniqueInner();
   }
 }
 

--- a/layout/style/StyleSheet.cpp
+++ b/layout/style/StyleSheet.cpp
@@ -29,29 +29,30 @@ StyleSheet::StyleSheet(StyleBackendType 
   , mDisabled(false)
   , mDocumentAssociationMode(NotOwnedByDocument)
   , mInner(nullptr)
   , mDirty(false)
 {
 }
 
 StyleSheet::StyleSheet(const StyleSheet& aCopy,
+                       StyleSheet* aParentToUse,
                        dom::CSSImportRule* aOwnerRuleToUse,
                        nsIDocument* aDocumentToUse,
                        nsINode* aOwningNodeToUse)
-  : mParent(nullptr)
+  : mParent(aParentToUse)
   , mTitle(aCopy.mTitle)
   , mDocument(aDocumentToUse)
   , mOwningNode(aOwningNodeToUse)
   , mOwnerRule(aOwnerRuleToUse)
   , mParsingMode(aCopy.mParsingMode)
   , mType(aCopy.mType)
   , mDisabled(aCopy.mDisabled)
-    // We only use this constructor during cloning.  It's the cloner's
-    // responsibility to notify us if we end up being owned by a document.
+  // We only use this constructor during cloning.  It's the cloner's
+  // responsibility to notify us if we end up being owned by a document.
   , mDocumentAssociationMode(NotOwnedByDocument)
   , mInner(aCopy.mInner) // Shallow copy, but concrete subclasses will fix up.
   , mDirty(aCopy.mDirty)
 {
   MOZ_ASSERT(mInner, "Should only copy StyleSheets with an mInner.");
   mInner->AddSheet(this);
 
   if (aCopy.mMedia) {

--- a/layout/style/StyleSheet.h
+++ b/layout/style/StyleSheet.h
@@ -48,16 +48,17 @@ class Rule;
  */
 class StyleSheet : public nsIDOMCSSStyleSheet
                  , public nsICSSLoaderObserver
                  , public nsWrapperCache
 {
 protected:
   StyleSheet(StyleBackendType aType, css::SheetParsingMode aParsingMode);
   StyleSheet(const StyleSheet& aCopy,
+             StyleSheet* aParentToUse,
              dom::CSSImportRule* aOwnerRuleToUse,
              nsIDocument* aDocumentToUse,
              nsINode* aOwningNodeToUse);
   virtual ~StyleSheet();
 
 public:
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
   NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_AMBIGUOUS(StyleSheet,