--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -1331,17 +1331,17 @@ nsIDocument::nsIDocument()
     mDidDocumentOpen(false),
     mHasDisplayDocument(false),
     mFontFaceSetDirty(true),
     mGetUserFontSetCalled(false),
     mPostedFlushUserFontSet(false),
     mEverInForeground(false),
     mCompatMode(eCompatibility_FullStandards),
     mReadyState(ReadyState::READYSTATE_UNINITIALIZED),
-    mStyleBackendType(mozilla::StyleBackendType::Gecko),
+    mStyleBackendType(StyleBackendType::None),
 #ifdef MOZILLA_INTERNAL_API
     mVisibilityState(dom::VisibilityState::Hidden),
 #else
     mDummy(0),
 #endif
     mType(eUnknown),
     mDefaultElementType(0),
     mAllowXULXBL(eTriUnset),
@@ -12923,17 +12923,17 @@ nsIDocument::ReportHasScrollLinkedEffect
                                   NS_LITERAL_CSTRING("Async Pan/Zoom"),
                                   this, nsContentUtils::eLAYOUT_PROPERTIES,
                                   "ScrollLinkedEffectFound2");
 }
 
 void
 nsIDocument::UpdateStyleBackendType()
 {
-  MOZ_ASSERT(mStyleBackendType == StyleBackendType(0),
+  MOZ_ASSERT(mStyleBackendType == StyleBackendType::None,
              "no need to call UpdateStyleBackendType now");
 
   // Assume Gecko by default.
   mStyleBackendType = StyleBackendType::Gecko;
 
 #ifdef MOZ_STYLO
   // XXX For now we use a Servo-backed style set only for (X)HTML documents
   // in content docshells.  This should let us avoid implementing XUL-specific

--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -1204,20 +1204,20 @@ public:
   /**
    * Get this document's CSSLoader.  This is guaranteed to not return null.
    */
   mozilla::css::Loader* CSSLoader() const {
     return mCSSLoader;
   }
 
   mozilla::StyleBackendType GetStyleBackendType() const {
-    if (mStyleBackendType == mozilla::StyleBackendType(0)) {
+    if (mStyleBackendType == mozilla::StyleBackendType::None) {
       const_cast<nsIDocument*>(this)->UpdateStyleBackendType();
     }
-    MOZ_ASSERT(mStyleBackendType != mozilla::StyleBackendType(0));
+    MOZ_ASSERT(mStyleBackendType != mozilla::StyleBackendType::None);
     return mStyleBackendType;
   }
 
   void UpdateStyleBackendType();
 
   bool IsStyledByServo() const {
     return GetStyleBackendType() == mozilla::StyleBackendType::Servo;
   }

--- a/layout/style/StyleBackendType.h
+++ b/layout/style/StyleBackendType.h
@@ -9,15 +9,16 @@
 
 namespace mozilla {
 
 /**
  * Enumeration that represents one of the two supported style system backends.
  */
 enum class StyleBackendType : uint8_t
 {
-  Gecko = 1,
+  None,
+  Gecko,
   Servo
 };
 
 } // namespace mozilla
 
 #endif // mozilla_StyleBackendType_h