--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -738,17 +738,17 @@ ServoStyleSet::ResolveXULTreePseudoStyle
 
 // manage the set of style sheets in the style set
 nsresult
 ServoStyleSet::AppendStyleSheet(SheetType aType,
                                 ServoStyleSheet* aSheet)
 {
   MOZ_ASSERT(aSheet);
   MOZ_ASSERT(aSheet->IsApplicable());
-  MOZ_ASSERT(nsStyleSet::IsCSSSheetType(aType));
+  MOZ_ASSERT(IsCSSSheetType(aType));
   MOZ_ASSERT(aSheet->RawContents(), "Raw sheet should be in place before insertion.");
 
   RemoveSheetOfType(aType, aSheet);
   AppendSheetOfType(aType, aSheet);
 
   if (mRawSet) {
     // Maintain a mirrored list of sheets on the servo side.
     // Servo will remove aSheet from its original position as part of the call
@@ -761,17 +761,17 @@ ServoStyleSet::AppendStyleSheet(SheetTyp
 }
 
 nsresult
 ServoStyleSet::PrependStyleSheet(SheetType aType,
                                  ServoStyleSheet* aSheet)
 {
   MOZ_ASSERT(aSheet);
   MOZ_ASSERT(aSheet->IsApplicable());
-  MOZ_ASSERT(nsStyleSet::IsCSSSheetType(aType));
+  MOZ_ASSERT(IsCSSSheetType(aType));
   MOZ_ASSERT(aSheet->RawContents(),
              "Raw sheet should be in place before insertion.");
 
   RemoveSheetOfType(aType, aSheet);
   PrependSheetOfType(aType, aSheet);
 
   if (mRawSet) {
     // Maintain a mirrored list of sheets on the servo side.
@@ -784,17 +784,17 @@ ServoStyleSet::PrependStyleSheet(SheetTy
   return NS_OK;
 }
 
 nsresult
 ServoStyleSet::RemoveStyleSheet(SheetType aType,
                                 ServoStyleSheet* aSheet)
 {
   MOZ_ASSERT(aSheet);
-  MOZ_ASSERT(nsStyleSet::IsCSSSheetType(aType));
+  MOZ_ASSERT(IsCSSSheetType(aType));
 
   RemoveSheetOfType(aType, aSheet);
   if (mRawSet) {
     // Maintain a mirrored list of sheets on the servo side.
     Servo_StyleSet_RemoveStyleSheet(mRawSet.get(), aSheet);
     SetStylistStyleSheetsDirty();
   }
 
@@ -858,24 +858,24 @@ ServoStyleSet::InsertStyleSheetBefore(Sh
   }
 
   return NS_OK;
 }
 
 int32_t
 ServoStyleSet::SheetCount(SheetType aType) const
 {
-  MOZ_ASSERT(nsStyleSet::IsCSSSheetType(aType));
+  MOZ_ASSERT(IsCSSSheetType(aType));
   return mSheets[aType].Length();
 }
 
 ServoStyleSheet*
 ServoStyleSet::StyleSheetAt(SheetType aType, int32_t aIndex) const
 {
-  MOZ_ASSERT(nsStyleSet::IsCSSSheetType(aType));
+  MOZ_ASSERT(IsCSSSheetType(aType));
   return mSheets[aType][aIndex];
 }
 
 void
 ServoStyleSet::AppendAllXBLStyleSheets(nsTArray<StyleSheet*>& aArray) const
 {
   if (mBindingManager) {
     mBindingManager->AppendAllSheets(aArray);

--- a/layout/style/SheetType.h
+++ b/layout/style/SheetType.h
@@ -18,21 +18,38 @@ namespace mozilla {
 //
 // Be sure to update NS_RULE_NODE_LEVEL_MASK when changing the number
 // of sheet types; static assertions enforce this.
 enum class SheetType : uint8_t {
   Agent, // CSS
   User, // CSS
   PresHint,
   Doc, // CSS
-  ScopedDoc,
+  ScopedDoc, // CSS
   StyleAttr,
   Override, // CSS
   Animation,
   Transition,
 
   Count,
   Unknown = 0xff
 };
 
+// Returns whether aSheetType represents a level of the cascade that uses
+// a StyleSheet object.
+inline bool
+IsCSSSheetType(SheetType aSheetType)
+{
+  switch (aSheetType) {
+    case SheetType::Agent:
+    case SheetType::User:
+    case SheetType::Doc:
+    case SheetType::ScopedDoc:
+    case SheetType::Override:
+      return true;
+    default:
+      return false;
+  }
+}
+
 } // namespace mozilla
 
 #endif // mozilla_SheetType_h

--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -194,27 +194,16 @@ static const SheetType gCSSSheetTypes[] 
   // From lowest to highest in cascading order.
   SheetType::Agent,
   SheetType::User,
   SheetType::Doc,
   SheetType::ScopedDoc,
   SheetType::Override
 };
 
-/* static */ bool
-nsStyleSet::IsCSSSheetType(SheetType aSheetType)
-{
-  for (SheetType type : gCSSSheetTypes) {
-    if (type == aSheetType) {
-      return true;
-    }
-  }
-  return false;
-}
-
 nsStyleSet::nsStyleSet()
   : mRuleTree(nullptr),
     mBatching(0),
     mStylesHaveChanged(0),
     mInShutdown(false),
     mInGC(false),
     mAuthorStyleDisabled(false),
     mInReconstruct(false),

--- a/layout/style/nsStyleSet.h
+++ b/layout/style/nsStyleSet.h
@@ -491,21 +491,16 @@ class nsStyleSet final
   nsIStyleRule* InitialStyleRule();
 
   bool HasRuleProcessorUsedByMultipleStyleSets(mozilla::SheetType aSheetType);
 
   // Tells the RestyleManager for the document using this style set
   // to drop any nsCSSSelector pointers it has.
   void ClearSelectors();
 
-  // Returns whether aSheetType represents a level of the cascade that uses
-  // CSSStyleSheets.  See gCSSSheetTypes in nsStyleSet.cpp for the list
-  // of CSS sheet types.
-  static bool IsCSSSheetType(mozilla::SheetType aSheetType);
-
   void SetUsesViewportUnits(bool aValue) {
     mUsesViewportUnits = aValue;
   }
 
 private:
   nsStyleSet(const nsStyleSet& aCopy) = delete;
   nsStyleSet& operator=(const nsStyleSet& aCopy) = delete;