--- a/layout/style/CSSStyleSheet.cpp
+++ b/layout/style/CSSStyleSheet.cpp
@@ -187,17 +187,16 @@ CSSStyleSheet::SizeOfIncludingThis(Mallo
     if (s->Inner()->mSheets.LastElement() == s) {
       n += s->Inner()->SizeOfIncludingThis(aMallocSizeOf);
     }
 
     // Measurement of the following members may be added later if DMD finds it
     // is worthwhile:
     // - s->mRuleCollection
     // - s->mRuleProcessors
-    // - s->mStyleSets
     //
     // The following members are not measured:
     // - s->mOwnerRule, because it's non-owning
     // - s->mScopeElement, because it's non-owning
 
     s = s->mNext ? s->mNext->AsGecko() : nullptr;
   }
   return n;
@@ -477,31 +476,16 @@ CSSStyleSheet::DropRuleProcessor(nsCSSRu
 {
   if (!mRuleProcessors)
     return NS_ERROR_FAILURE;
   return mRuleProcessors->RemoveElement(aProcessor)
            ? NS_OK
            : NS_ERROR_FAILURE;
 }
 
-void
-CSSStyleSheet::AddStyleSet(nsStyleSet* aStyleSet)
-{
-  NS_ASSERTION(!mStyleSets.Contains(aStyleSet),
-               "style set already registered");
-  mStyleSets.AppendElement(aStyleSet);
-}
-
-void
-CSSStyleSheet::DropStyleSet(nsStyleSet* aStyleSet)
-{
-  DebugOnly<bool> found = mStyleSets.RemoveElement(aStyleSet);
-  NS_ASSERTION(found, "didn't find style set");
-}
-
 bool
 CSSStyleSheet::UseForPresentation(nsPresContext* aPresContext,
                                   nsMediaQueryResultCacheKey& aKey) const
 {
   if (mMedia) {
     MOZ_ASSERT(aPresContext);
     auto media = static_cast<nsMediaList*>(mMedia.get());
     return media->Matches(aPresContext, &aKey);

--- a/layout/style/CSSStyleSheet.h
+++ b/layout/style/CSSStyleSheet.h
@@ -7,17 +7,16 @@
 /* representation of a CSS style sheet */
 
 #ifndef mozilla_CSSStyleSheet_h
 #define mozilla_CSSStyleSheet_h
 
 #include "mozilla/Attributes.h"
 #include "mozilla/IncrementalClearCOMRuleArray.h"
 #include "mozilla/MemoryReporting.h"
-#include "mozilla/StyleSetHandle.h"
 #include "mozilla/StyleSheet.h"
 #include "mozilla/StyleSheetInfo.h"
 #include "mozilla/css/SheetParsingMode.h"
 
 #include "nscore.h"
 #include "nsCOMPtr.h"
 #include "nsAutoPtr.h"
 #include "nsTArrayForwardDeclare.h"
@@ -26,17 +25,16 @@
 #include "nsCycleCollectionParticipant.h"
 #include "mozilla/net/ReferrerPolicy.h"
 #include "mozilla/dom/SRIMetadata.h"
 
 class CSSRuleListImpl;
 class nsCSSRuleProcessor;
 class nsIURI;
 class nsMediaQueryResultCacheKey;
-class nsStyleSet;
 class nsPresContext;
 class nsXMLNameSpaceMap;
 
 namespace mozilla {
 class CSSStyleSheet;
 struct ChildSheetListBuilder;
 
 namespace css {
@@ -133,19 +131,16 @@ public:
     NS_ASSERTION(mDirty,
                  "sheet must be marked dirty before handing out child rules");
     DidDirty();
   }
 
   nsresult AddRuleProcessor(nsCSSRuleProcessor* aProcessor);
   nsresult DropRuleProcessor(nsCSSRuleProcessor* aProcessor);
 
-  void AddStyleSet(nsStyleSet* aStyleSet);
-  void DropStyleSet(nsStyleSet* aStyleSet);
-
   // nsICSSLoaderObserver interface
   NS_IMETHOD StyleSheetLoaded(StyleSheet* aSheet, bool aWasAlternate,
                               nsresult aStatus) override;
 
   void EnsureUniqueInner() override;
 
   // Append all of this sheet's child sheets to aArray.
   void AppendAllChildSheets(nsTArray<CSSStyleSheet*>& aArray);
@@ -220,17 +215,16 @@ protected:
 
   css::ImportRule*      mOwnerRule; // weak ref
 
   RefPtr<CSSRuleListImpl> mRuleCollection;
   bool                  mInRuleProcessorCache;
   RefPtr<dom::Element> mScopeElement;
 
   AutoTArray<nsCSSRuleProcessor*, 8>* mRuleProcessors;
-  nsTArray<StyleSetHandle> mStyleSets;
 
   friend class mozilla::StyleSheet;
   friend class ::nsCSSRuleProcessor;
 };
 
 NS_DEFINE_STATIC_IID_ACCESSOR(CSSStyleSheet, NS_CSS_STYLE_SHEET_IMPL_CID)
 
 } // namespace mozilla

--- a/layout/style/StyleSheet.cpp
+++ b/layout/style/StyleSheet.cpp
@@ -402,16 +402,31 @@ void
 StyleSheet::WillDirty()
 {
   if (mInner->mComplete) {
     EnsureUniqueInner();
   }
 }
 
 void
+StyleSheet::AddStyleSet(StyleSetHandle aStyleSet)
+{
+  NS_ASSERTION(!mStyleSets.Contains(aStyleSet),
+               "style set already registered");
+  mStyleSets.AppendElement(aStyleSet);
+}
+
+void
+StyleSheet::DropStyleSet(StyleSetHandle aStyleSet)
+{
+  DebugOnly<bool> found = mStyleSets.RemoveElement(aStyleSet);
+  NS_ASSERTION(found, "didn't find style set");
+}
+
+void
 StyleSheet::EnsureUniqueInner()
 {
   MOZ_ASSERT(mInner->mSheets.Length() != 0,
              "unexpected number of outers");
   mDirty = true;
 
   if (mInner->mSheets.Length() == 1) {
     // already unique
@@ -670,16 +685,17 @@ StyleSheet::SizeOfIncludingThis(MallocSi
   const StyleSheet* s = this;
   while (s) {
     n += aMallocSizeOf(s);
 
     // Measurement of the following members may be added later if DMD finds it
     // is worthwhile:
     // - s->mTitle
     // - s->mMedia
+    // - s->mStyleSets
 
     s = s->mNext;
   }
   return n;
 }
 
 #ifdef DEBUG
 void

--- a/layout/style/StyleSheet.h
+++ b/layout/style/StyleSheet.h
@@ -8,30 +8,32 @@
 #define mozilla_StyleSheet_h
 
 #include "mozilla/css/SheetParsingMode.h"
 #include "mozilla/dom/CSSStyleSheetBinding.h"
 #include "mozilla/net/ReferrerPolicy.h"
 #include "mozilla/StyleBackendType.h"
 #include "mozilla/CORSMode.h"
 #include "mozilla/ServoUtils.h"
+#include "mozilla/StyleSetHandle.h"
 
 #include "nsICSSLoaderObserver.h"
 #include "nsIDOMCSSStyleSheet.h"
 #include "nsWrapperCache.h"
 
 class nsIDocument;
 class nsINode;
 class nsIPrincipal;
 class nsCSSRuleProcessor;
 
 namespace mozilla {
 
 class CSSStyleSheet;
 class ServoStyleSheet;
+class StyleSetHandle;
 struct StyleSheetInfo;
 struct CSSStyleSheetInner;
 
 namespace dom {
 class CSSRuleList;
 class MediaList;
 class SRIMetadata;
 } // namespace dom
@@ -213,16 +215,19 @@ public:
   NS_IMETHOD DeleteRule(uint32_t aIndex) final;
 
   // Changes to sheets should be inside of a WillDirty-DidDirty pair.
   // However, the calls do not need to be matched; it's ok to call
   // WillDirty and then make no change and skip the DidDirty call.
   void WillDirty();
   virtual void DidDirty() {}
 
+  void AddStyleSet(StyleSetHandle aStyleSet);
+  void DropStyleSet(StyleSetHandle aStyleSet);
+
   nsresult DeleteRuleFromGroup(css::GroupRule* aGroup, uint32_t aIndex);
   nsresult InsertRuleIntoGroup(const nsAString& aRule,
                                css::GroupRule* aGroup, uint32_t aIndex);
 
 private:
   // Get a handle to the various stylesheet bits which live on the 'inner' for
   // gecko stylesheets and live on the StyleSheet for Servo stylesheets.
   inline StyleSheetInfo& SheetInfo();
@@ -290,16 +295,18 @@ protected:
   DocumentAssociationMode mDocumentAssociationMode;
 
   // Core information we get from parsed sheets, which are shared amongst
   // StyleSheet clones.
   StyleSheetInfo* mInner;
 
   bool mDirty; // has been modified
 
+  nsTArray<StyleSetHandle> mStyleSets;
+
   friend class ::nsCSSRuleProcessor;
 
   // Make CSSStyleSheet and ServoStyleSheet friends so they can access
   // protected members of other StyleSheet objects (useful for iterating
   // through children).
   friend class mozilla::CSSStyleSheet;
   friend class mozilla::ServoStyleSheet;
 

--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -591,70 +591,70 @@ nsStyleSet::AppendStyleSheet(SheetType a
 {
   NS_PRECONDITION(aSheet, "null arg");
   NS_ASSERTION(aSheet->IsApplicable(),
                "Inapplicable sheet being placed in style set");
   bool present = mSheets[aType].RemoveElement(aSheet);
   mSheets[aType].AppendElement(aSheet);
 
   if (!present && IsCSSSheetType(aType)) {
-    aSheet->AddStyleSet(this);
+    aSheet->AddStyleSet(StyleSetHandle(this));
   }
 
   return DirtyRuleProcessors(aType);
 }
 
 nsresult
 nsStyleSet::PrependStyleSheet(SheetType aType, CSSStyleSheet* aSheet)
 {
   NS_PRECONDITION(aSheet, "null arg");
   NS_ASSERTION(aSheet->IsApplicable(),
                "Inapplicable sheet being placed in style set");
   bool present = mSheets[aType].RemoveElement(aSheet);
   mSheets[aType].InsertElementAt(0, aSheet);
 
   if (!present && IsCSSSheetType(aType)) {
-    aSheet->AddStyleSet(this);
+    aSheet->AddStyleSet(StyleSetHandle(this));
   }
 
   return DirtyRuleProcessors(aType);
 }
 
 nsresult
 nsStyleSet::RemoveStyleSheet(SheetType aType, CSSStyleSheet* aSheet)
 {
   NS_PRECONDITION(aSheet, "null arg");
   NS_ASSERTION(aSheet->IsComplete(),
                "Incomplete sheet being removed from style set");
   if (mSheets[aType].RemoveElement(aSheet)) {
     if (IsCSSSheetType(aType)) {
-      aSheet->DropStyleSet(this);
+      aSheet->DropStyleSet(StyleSetHandle(this));
     }
   }
 
   return DirtyRuleProcessors(aType);
 }
 
 nsresult
 nsStyleSet::ReplaceSheets(SheetType aType,
                           const nsTArray<RefPtr<CSSStyleSheet>>& aNewSheets)
 {
   bool cssSheetType = IsCSSSheetType(aType);
   if (cssSheetType) {
     for (CSSStyleSheet* sheet : mSheets[aType]) {
-      sheet->DropStyleSet(this);
+      sheet->DropStyleSet(StyleSetHandle(this));
     }
   }
 
   mSheets[aType].Clear();
   mSheets[aType].AppendElements(aNewSheets);
 
   if (cssSheetType) {
     for (CSSStyleSheet* sheet : mSheets[aType]) {
-      sheet->AddStyleSet(this);
+      sheet->AddStyleSet(StyleSetHandle(this));
     }
   }
 
   return DirtyRuleProcessors(aType);
 }
 
 nsresult
 nsStyleSet::InsertStyleSheetBefore(SheetType aType, CSSStyleSheet* aNewSheet,
@@ -667,17 +667,17 @@ nsStyleSet::InsertStyleSheetBefore(Sheet
   bool present = mSheets[aType].RemoveElement(aNewSheet);
   int32_t idx = mSheets[aType].IndexOf(aReferenceSheet);
   if (idx < 0)
     return NS_ERROR_INVALID_ARG;
 
   mSheets[aType].InsertElementAt(idx, aNewSheet);
 
   if (!present && IsCSSSheetType(aType)) {
-    aNewSheet->AddStyleSet(this);
+    aNewSheet->AddStyleSet(StyleSetHandle(this));
   }
 
   return DirtyRuleProcessors(aType);
 }
 
 static inline uint32_t
 DirtyBit(SheetType aType)
 {
@@ -729,17 +729,17 @@ nsStyleSet::AddDocStyleSheet(CSSStyleShe
   nsTArray<RefPtr<CSSStyleSheet>>& sheets = mSheets[type];
 
   bool present = sheets.RemoveElement(aSheet);
 
   size_t index = aDocument->FindDocStyleSheetInsertionPoint(sheets, aSheet);
   sheets.InsertElementAt(index, aSheet);
 
   if (!present) {
-    aSheet->AddStyleSet(this);
+    aSheet->AddStyleSet(StyleSetHandle(this));
   }
 
   return DirtyRuleProcessors(type);
 }
 
 void
 nsStyleSet::AppendAllXBLStyleSheets(nsTArray<mozilla::CSSStyleSheet*>& aArray) const
 {