--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -1470,16 +1470,17 @@ nsIDocument::nsIDocument()
     mHasScrollLinkedEffect(false),
     mFrameRequestCallbacksScheduled(false),
     mIsTopLevelContentDocument(false),
     mIsContentDocument(false),
     mDidCallBeginLoad(false),
     mBufferingCSPViolations(false),
     mAllowPaymentRequest(false),
     mEncodingMenuDisabled(false),
+    mIsShadowDOMEnabled(false),
     mIsSVGGlyphsDocument(false),
     mAllowUnsafeHTML(false),
     mIsScopedStyleEnabled(eScopedStyle_Unknown),
     mCompatMode(eCompatibility_FullStandards),
     mReadyState(ReadyState::READYSTATE_UNINITIALIZED),
     mStyleBackendType(StyleBackendType::None),
 #ifdef MOZILLA_INTERNAL_API
     mVisibilityState(dom::VisibilityState::Hidden),
@@ -1513,20 +1514,16 @@ nsIDocument::nsIDocument()
     mServoRestyleRootDirtyBits(0),
     mThrowOnDynamicMarkupInsertionCounter(0),
     mIgnoreOpensDuringUnloadCounter(0)
 {
   SetIsInDocument();
   for (auto& cnt : mIncCounters) {
     cnt = 0;
   }
-
-  // Set this when document is created and value stays the same for the lifetime
-  // of the document.
-  mIsShadowDOMEnabled = nsContentUtils::IsShadowDOMEnabled();
 }
 
 nsDocument::nsDocument(const char* aContentType)
   : nsIDocument()
   , mSubDocuments(nullptr)
   , mFlashClassification(FlashClassification::Unclassified)
   , mHeaderData(nullptr)
   , mIsGoingAway(false)
@@ -2161,16 +2158,24 @@ nsDocument::Init()
   // mNodeInfo keeps NodeInfoManager alive!
   mNodeInfo = mNodeInfoManager->GetDocumentNodeInfo();
   NS_ENSURE_TRUE(mNodeInfo, NS_ERROR_OUT_OF_MEMORY);
   MOZ_ASSERT(mNodeInfo->NodeType() == DOCUMENT_NODE,
              "Bad NodeType in aNodeInfo");
 
   NS_ASSERTION(OwnerDoc() == this, "Our nodeinfo is busted!");
 
+  UpdateStyleBackendType();
+
+  // Set this when document is initialized and value stays the same for the
+  // lifetime of the document.
+  mIsShadowDOMEnabled =
+    mStyleBackendType == StyleBackendType::Servo &&
+    nsContentUtils::IsShadowDOMEnabled();
+
   // If after creation the owner js global is not set for a document
   // we use the default compartment for this document, instead of creating
   // wrapper in some random compartment when the document is exposed to js
   // via some events.
   nsCOMPtr<nsIGlobalObject> global = xpc::NativeGlobal(xpc::PrivilegedJunkScope());
   NS_ENSURE_TRUE(global, NS_ERROR_FAILURE);
   mScopeObject = do_GetWeakReference(global);
   MOZ_ASSERT(mScopeObject);

--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -1398,20 +1398,18 @@ 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::None) {
-      const_cast<nsIDocument*>(this)->UpdateStyleBackendType();
-    }
-    MOZ_ASSERT(mStyleBackendType != mozilla::StyleBackendType::None);
+    MOZ_ASSERT(mStyleBackendType != mozilla::StyleBackendType::None,
+               "Not initialized yet");
     return mStyleBackendType;
   }
 
   /**
    * Documents generally decide their style backend type themselves, and
    * this is only used for XBL documents to set their style backend type to
    * their bounding document's.
    */