diff --git a/layout/style/CSSStyleSheet.cpp b/layout/style/CSSStyleSheet.cpp
index dc99e9ec6bfd..92d9d2774ae5 100644
--- a/layout/style/CSSStyleSheet.cpp
+++ b/layout/style/CSSStyleSheet.cpp
@@ -208,11 +208,9 @@ CSSStyleSheet::RebuildChildList(css::Rule* aRule, void* aBuilder)
 size_t
 CSSStyleSheet::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
 {
-  size_t n = 0;
+  size_t n = StyleSheet::SizeOfIncludingThis(aMallocSizeOf);
   const CSSStyleSheet* s = this;
   while (s) {
-    n += aMallocSizeOf(s);
-
     // Each inner can be shared by multiple sheets.  So we only count the inner
     // if this sheet is the last one in the list of those sharing it.  As a
     // result, the last such sheet takes all the blame for the memory
@@ -226,8 +224,6 @@ CSSStyleSheet::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
 
     // Measurement of the following members may be added later if DMD finds it
     // is worthwhile:
-    // - s->mTitle
-    // - s->mMedia
     // - s->mRuleCollection
     // - s->mRuleProcessors
     //
diff --git a/layout/style/CSSStyleSheet.h b/layout/style/CSSStyleSheet.h
index 45440d6d4b0f..76d7031cde33 100644
--- a/layout/style/CSSStyleSheet.h
+++ b/layout/style/CSSStyleSheet.h
@@ -169,7 +169,7 @@ public:
   // list after we clone a unique inner for ourselves.
   static bool RebuildChildList(css::Rule* aRule, void* aBuilder);
 
-  size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const;
+  size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const override;
 
   dom::Element* GetScopeElement() const { return mScopeElement; }
   void SetScopeElement(dom::Element* aScopeElement)
diff --git a/layout/style/ServoStyleSheet.cpp b/layout/style/ServoStyleSheet.cpp
index 5c2a62f321a7..8d7e4c4fecb6 100644
--- a/layout/style/ServoStyleSheet.cpp
+++ b/layout/style/ServoStyleSheet.cpp
@@ -109,12 +109,6 @@ ServoStyleSheet::DropRuleList()
   }
 }
 
-size_t
-ServoStyleSheet::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
-{
-  MOZ_CRASH("stylo: not implemented");
-}
-
 css::Rule*
 ServoStyleSheet::GetDOMOwnerRule() const
 {
diff --git a/layout/style/ServoStyleSheet.h b/layout/style/ServoStyleSheet.h
index f1ec541c712f..fbf56e1005a6 100644
--- a/layout/style/ServoStyleSheet.h
+++ b/layout/style/ServoStyleSheet.h
@@ -53,8 +53,6 @@ public:
    */
   void LoadFailed();
 
-  size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const;
-
   RawServoStyleSheet* RawSheet() const { return mSheet; }
   void SetSheetForImport(RawServoStyleSheet* aSheet) {
     MOZ_ASSERT(!mSheet);
diff --git a/layout/style/StyleSheet.cpp b/layout/style/StyleSheet.cpp
index 382435c65d1d..ec7aeaac8d50 100644
--- a/layout/style/StyleSheet.cpp
+++ b/layout/style/StyleSheet.cpp
@@ -432,6 +432,24 @@ StyleSheet::AppendStyleSheet(StyleSheet* aSheet)
   DidDirty();
 }
 
+size_t
+StyleSheet::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
+{
+  size_t n = 0;
+  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 = s->mNext;
+  }
+  return n;
+}
+
 #ifdef DEBUG
 void
 StyleSheet::List(FILE* out, int32_t aIndent) const
diff --git a/layout/style/StyleSheet.h b/layout/style/StyleSheet.h
index bfa4eb82999f..dd73ddeaefa6 100644
--- a/layout/style/StyleSheet.h
+++ b/layout/style/StyleSheet.h
@@ -151,7 +151,7 @@ public:
   // Get this style sheet's integrity metadata
   inline void GetIntegrity(dom::SRIMetadata& aResult) const;
 
-  inline size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const;
+  virtual size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const;
 #ifdef DEBUG
   virtual void List(FILE* aOut = stdout, int32_t aIndex = 0) const;
 #endif
diff --git a/layout/style/StyleSheetInlines.h b/layout/style/StyleSheetInlines.h
index 3f83798b0f67..c6aa7ad353c8 100644
--- a/layout/style/StyleSheetInlines.h
+++ b/layout/style/StyleSheetInlines.h
@@ -134,12 +134,6 @@ StyleSheet::GetIntegrity(dom::SRIMetadata& aResult) const
   aResult = SheetInfo().mIntegrity;
 }
 
-size_t
-StyleSheet::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
-{
-  MOZ_STYLO_FORWARD(SizeOfIncludingThis, (aMallocSizeOf))
-}
-
 void StyleSheet::WillDirty() { MOZ_STYLO_FORWARD(WillDirty, ()) }
 void StyleSheet::DidDirty() { MOZ_STYLO_FORWARD(DidDirty, ()) }