--- a/dom/base/nsIDocumentInlines.h
+++ b/dom/base/nsIDocumentInlines.h
@@ -1,16 +1,17 @@
 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 #ifndef nsIDocumentInlines_h
 #define nsIDocumentInlines_h
 
+#include "nsContentUtils.h"
 #include "nsIDocument.h"
 #include "mozilla/dom/HTMLBodyElement.h"
 #include "nsStyleSheetService.h"
 
 inline mozilla::dom::HTMLBodyElement*
 nsIDocument::GetBodyElement()
 {
   return static_cast<mozilla::dom::HTMLBodyElement*>(GetHtmlChildElement(nsGkAtoms::body));
@@ -59,15 +60,18 @@ nsIDocument::FindDocStyleSheetInsertionP
 
 inline void
 nsIDocument::SetServoRestyleRoot(nsINode* aRoot, uint32_t aDirtyBits)
 {
   MOZ_ASSERT(aRoot);
   MOZ_ASSERT(aDirtyBits);
   MOZ_ASSERT((aDirtyBits & ~Element::kAllServoDescendantBits) == 0);
 
+  MOZ_ASSERT(!mServoRestyleRoot ||
+             mServoRestyleRoot == aRoot ||
+             nsContentUtils::ContentIsFlattenedTreeDescendantOfForStyle(mServoRestyleRoot, aRoot));
   MOZ_ASSERT(aRoot == aRoot->OwnerDocAsNode() ||
              (aRoot->IsElement() && aRoot->IsInComposedDoc()));
   mServoRestyleRoot = aRoot;
   mServoRestyleRootDirtyBits = aDirtyBits;
 }
 
 #endif // nsIDocumentInlines_h