diff --git a/layout/style/CSSStyleSheet.cpp b/layout/style/CSSStyleSheet.cpp
index 4833bce7c89f..939c1ea3b243 100644
--- a/layout/style/CSSStyleSheet.cpp
+++ b/layout/style/CSSStyleSheet.cpp
@@ -371,7 +371,6 @@ CSSStyleSheetInner::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
 CSSStyleSheet::CSSStyleSheet(css::SheetParsingMode aParsingMode,
                              CORSMode aCORSMode, ReferrerPolicy aReferrerPolicy)
   : StyleSheet(StyleBackendType::Gecko, aParsingMode),
-    mParent(nullptr),
     mOwnerRule(nullptr),
     mDirty(false),
     mInRuleProcessorCache(false),
@@ -387,7 +386,6 @@ CSSStyleSheet::CSSStyleSheet(css::SheetParsingMode aParsingMode,
                              ReferrerPolicy aReferrerPolicy,
                              const SRIMetadata& aIntegrity)
   : StyleSheet(StyleBackendType::Gecko, aParsingMode),
-    mParent(nullptr),
     mOwnerRule(nullptr),
     mDirty(false),
     mInRuleProcessorCache(false),
@@ -404,7 +402,6 @@ CSSStyleSheet::CSSStyleSheet(const CSSStyleSheet& aCopy,
                              nsIDocument* aDocumentToUse,
                              nsINode* aOwningNodeToUse)
   : StyleSheet(aCopy, aDocumentToUse, aOwningNodeToUse),
-    mParent(aParentToUse),
     mOwnerRule(aOwnerRuleToUse),
     mDirty(aCopy.mDirty),
     mInRuleProcessorCache(false),
@@ -412,6 +409,7 @@ CSSStyleSheet::CSSStyleSheet(const CSSStyleSheet& aCopy,
     mInner(aCopy.mInner),
     mRuleProcessors(nullptr)
 {
+  mParent = aParentToUse;
 
   mInner->AddSheet(this);
 
@@ -609,12 +607,6 @@ CSSStyleSheet::EnabledStateChangedInternal()
   ClearRuleCascades();
 }
 
-CSSStyleSheet*
-CSSStyleSheet::GetParentSheet() const
-{
-  return mParent;
-}
-
 void
 CSSStyleSheet::SetAssociatedDocument(nsIDocument* aDocument,
                                      DocumentAssociationMode aAssociationMode)
@@ -657,7 +649,8 @@ CSSStyleSheet::FindOwningWindowInnerID() const
   }
 
   if (windowID == 0 && mParent) {
-    windowID = mParent->FindOwningWindowInnerID();
+    CSSStyleSheet* parentAsCSS = mParent->AsGecko();
+    windowID = parentAsCSS->FindOwningWindowInnerID();
   }
 
   return windowID;
diff --git a/layout/style/CSSStyleSheet.h b/layout/style/CSSStyleSheet.h
index 8ba8a83860a7..329cc7091941 100644
--- a/layout/style/CSSStyleSheet.h
+++ b/layout/style/CSSStyleSheet.h
@@ -115,8 +115,6 @@ public:
 
   bool HasRules() const;
 
-  // style sheet owner info
-  CSSStyleSheet* GetParentSheet() const;  // may be null
   void SetAssociatedDocument(nsIDocument* aDocument,
                              DocumentAssociationMode aAssociationMode);
 
@@ -236,7 +234,6 @@ protected:
   void EnabledStateChangedInternal();
 
   RefPtr<CSSStyleSheet> mNext;
-  CSSStyleSheet*        mParent;    // weak ref
   css::ImportRule*      mOwnerRule; // weak ref
 
   RefPtr<CSSRuleListImpl> mRuleCollection;
diff --git a/layout/style/ServoStyleSheet.cpp b/layout/style/ServoStyleSheet.cpp
index f6f34300cafa..fee88cf95b66 100644
--- a/layout/style/ServoStyleSheet.cpp
+++ b/layout/style/ServoStyleSheet.cpp
@@ -67,15 +67,6 @@ ServoStyleSheet::SetAssociatedDocument(nsIDocument* aDocument,
   mDocumentAssociationMode = aAssociationMode;
 }
 
-ServoStyleSheet*
-ServoStyleSheet::GetParentSheet() const
-{
-  // XXXheycam: When we implement support for child sheets, we'll have
-  // to fix SetAssociatedDocument to propagate the associated document down
-  // to the children.
-  MOZ_CRASH("stylo: not implemented");
-}
-
 void
 ServoStyleSheet::AppendStyleSheet(ServoStyleSheet* aSheet)
 {
diff --git a/layout/style/ServoStyleSheet.h b/layout/style/ServoStyleSheet.h
index 8411febbb543..48dee2029839 100644
--- a/layout/style/ServoStyleSheet.h
+++ b/layout/style/ServoStyleSheet.h
@@ -42,7 +42,6 @@ public:
   void SetAssociatedDocument(nsIDocument* aDocument,
                              DocumentAssociationMode aAssociationMode);
 
-  ServoStyleSheet* GetParentSheet() const;
   void AppendStyleSheet(ServoStyleSheet* aSheet);
 
   MOZ_MUST_USE nsresult ParseSheet(css::Loader* aLoader,
diff --git a/layout/style/StyleSheet.cpp b/layout/style/StyleSheet.cpp
index 81b002d346ed..5e499c4d0605 100644
--- a/layout/style/StyleSheet.cpp
+++ b/layout/style/StyleSheet.cpp
@@ -19,7 +19,8 @@
 namespace mozilla {
 
 StyleSheet::StyleSheet(StyleBackendType aType, css::SheetParsingMode aParsingMode)
-  : mDocument(nullptr)
+  : mParent(nullptr)
+  , mDocument(nullptr)
   , mOwningNode(nullptr)
   , mParsingMode(aParsingMode)
   , mType(aType)
@@ -31,7 +32,8 @@ StyleSheet::StyleSheet(StyleBackendType aType, css::SheetParsingMode aParsingMod
 StyleSheet::StyleSheet(const StyleSheet& aCopy,
                        nsIDocument* aDocumentToUse,
                        nsINode* aOwningNodeToUse)
-  : mTitle(aCopy.mTitle)
+  : mParent(nullptr)
+  , mTitle(aCopy.mTitle)
   , mDocument(aDocumentToUse)
   , mOwningNode(aOwningNodeToUse)
   , mParsingMode(aCopy.mParsingMode)
diff --git a/layout/style/StyleSheet.h b/layout/style/StyleSheet.h
index 1ee75cd680d0..74815f5bcabf 100644
--- a/layout/style/StyleSheet.h
+++ b/layout/style/StyleSheet.h
@@ -124,7 +124,7 @@ public:
                                     DocumentAssociationMode aMode);
   inline void ClearAssociatedDocument();
   nsINode* GetOwnerNode() const { return mOwningNode; }
-  inline StyleSheet* GetParentSheet() const;
+  inline StyleSheet* GetParentSheet() const { return mParent; }
 
   inline void AppendStyleSheet(StyleSheet* aSheet);
 
@@ -227,6 +227,8 @@ protected:
   // Called from SetEnabled when the enabled state changed.
   void EnabledStateChanged();
 
+  StyleSheet*           mParent;    // weak ref
+
   nsString              mTitle;
   nsIDocument*          mDocument; // weak ref; parents maintain this for their children
   nsINode*              mOwningNode; // weak ref
diff --git a/layout/style/StyleSheetInlines.h b/layout/style/StyleSheetInlines.h
index c0b8495f8757..43d8b4154c4c 100644
--- a/layout/style/StyleSheetInlines.h
+++ b/layout/style/StyleSheetInlines.h
@@ -96,12 +96,6 @@ StyleSheet::ClearAssociatedDocument()
   MOZ_STYLO_FORWARD(SetAssociatedDocument, (nullptr, NotOwnedByDocument));
 }
 
-StyleSheet*
-StyleSheet::GetParentSheet() const
-{
-  MOZ_STYLO_FORWARD(GetParentSheet, ())
-}
-
 StyleSheet*
 StyleSheet::GetParentStyleSheet() const
 {