--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -4382,16 +4382,19 @@ NoteDirtyElement(Element* aElement, uint
       // We didn't find a common ancestor element. That means we're descended
       // from two different document style roots, so the common ancestor is the
       // document.
       doc->SetServoRestyleRoot(doc, existingBits | aBit);
     }
   }
 
   MOZ_ASSERT(aElement == doc->GetServoRestyleRoot() ||
+             nsContentUtils::ContentIsFlattenedTreeDescendantOf(
+               aElement, doc->GetServoRestyleRoot()));
+  MOZ_ASSERT(aElement == doc->GetServoRestyleRoot() ||
              BitIsPropagated(parent, aBit, doc->GetServoRestyleRoot()));
   MOZ_ASSERT(doc->GetServoRestyleRootDirtyBits() & aBit);
 }
 
 void
 Element::NoteDirtyForServo()
 {
   NoteDirtyElement(this, ELEMENT_HAS_DIRTY_DESCENDANTS_FOR_SERVO);