--- a/dom/base/nsGkAtomList.h
+++ b/dom/base/nsGkAtomList.h
@@ -2069,16 +2069,17 @@ GK_ATOM(animationsOfAfterProperty, "Anim
 GK_ATOM(transitionsProperty, "TransitionsProperty")        // FrameTransitions*
 GK_ATOM(transitionsOfBeforeProperty, "TransitionsOfBeforeProperty") // FrameTransitions*
 GK_ATOM(transitionsOfAfterProperty, "TransitionsOfAfterProperty") // FrameTransitions*
 GK_ATOM(genConInitializerProperty, "QuoteNodeProperty")
 GK_ATOM(labelMouseDownPtProperty, "LabelMouseDownPtProperty")
 GK_ATOM(baseURIProperty, "baseURIProperty")
 GK_ATOM(lockedStyleStates, "lockedStyleStates")
 GK_ATOM(apzCallbackTransform, "apzCallbackTransform")
+GK_ATOM(restylableAnonymousNode, "restylableAnonymousNode")
 
 // Languages for lang-specific transforms
 GK_ATOM(Japanese, "ja")
 GK_ATOM(Chinese, "zh-CN")
 GK_ATOM(Taiwanese, "zh-TW")
 GK_ATOM(HongKongChinese, "zh-HK")
 GK_ATOM(Unicode, "x-unicode")
 

--- a/editor/libeditor/nsHTMLAnonymousUtils.cpp
+++ b/editor/libeditor/nsHTMLAnonymousUtils.cpp
@@ -183,16 +183,25 @@ nsHTMLEditor::CreateAnonymousElement(con
   }
 
   nsElementDeletionObserver* observer =
     new nsElementDeletionObserver(newContent, parentContent);
   NS_ADDREF(observer); // NodeWillBeDestroyed releases.
   parentContent->AddMutationObserver(observer);
   newContent->AddMutationObserver(observer);
 
+#ifdef DEBUG
+  // Editor anonymous content gets passed to RecreateFramesFor... which can't
+  // _really_ deal with anonymous content (because it can't get the frame tree
+  // ordering right).  But for us the ordering doesn't matter so this is sort of
+  // ok.
+  newContent->SetProperty(nsGkAtoms::restylableAnonymousNode,
+			  reinterpret_cast<void*>(true));
+#endif // DEBUG
+
   // display the element
   ps->RecreateFramesFor(newContent);
 
   newElement.forget(aReturn);
   return NS_OK;
 }
 
 // Removes event listener and calls DeleteRefToAnonymousNode.

--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -6550,17 +6550,18 @@ nsCSSFrameConstructor::GetInsertionPrevS
     if (aStartSkipChild) {
       iter.Seek(aStartSkipChild);
     } else {
       iter.Seek(aChild);
     }
   } else {
     // Prime the iterator for the call to FindPreviousSibling.
     iter.GetNextChild();
-    NS_WARNING("Someone passed native anonymous content directly into frame "
+    MOZ_ASSERT(aChild->GetProperty(nsGkAtoms::restylableAnonymousNode),
+               "Someone passed native anonymous content directly into frame "
                "construction.  Stop doing that!");
   }
 
   // Note that FindPreviousSibling is passed the iterator by value, so that
   // the later usage of the iterator starts from the same place.
   uint8_t childDisplay = UNSET_DISPLAY;
   nsIFrame* prevSibling =
     FindPreviousSibling(iter, iter.Get(), childDisplay, aInsertion->mParentFrame);

--- a/layout/generic/nsCanvasFrame.cpp
+++ b/layout/generic/nsCanvasFrame.cpp
@@ -140,16 +140,25 @@ nsCanvasFrame::CreateAnonymousContent(ns
                                                    mDummyTouchListener, false);
     mSelectionCaretsEndElement->AddEventListener(NS_LITERAL_STRING("touchstart"),
                                                  mDummyTouchListener, false);
     NS_ENSURE_SUCCESS(rv, rv);
   }
 
   // Create the custom content container.
   mCustomContentContainer = doc->CreateHTMLElement(nsGkAtoms::div);
+#ifdef DEBUG
+  // We restyle our mCustomContentContainer, even though it's root anonymous
+  // content.  Normally that's not OK because the frame constructor doesn't know
+  // how to order the frame tree in such cases, but we make this work for this
+  // particular case, so it's OK.
+  mCustomContentContainer->SetProperty(nsGkAtoms::restylableAnonymousNode,
+                                       reinterpret_cast<void*>(true));
+#endif // DEBUG
+
   aElements.AppendElement(mCustomContentContainer);
 
   // XXX add :moz-native-anonymous or will that be automatically set?
   rv = mCustomContentContainer->SetAttr(kNameSpaceID_None, nsGkAtoms::_class,
                                         NS_LITERAL_STRING("moz-custom-content-container"),
                                         true);
   NS_ENSURE_SUCCESS(rv, rv);
 

--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -3778,31 +3778,47 @@ ScrollFrameHelper::CreateAnonymousConten
   nodeInfo = nodeInfoManager->GetNodeInfo(nsGkAtoms::scrollbar, nullptr,
                                           kNameSpaceID_XUL,
                                           nsIDOMNode::ELEMENT_NODE);
   NS_ENSURE_TRUE(nodeInfo, NS_ERROR_OUT_OF_MEMORY);
 
   if (canHaveHorizontal) {
     nsRefPtr<NodeInfo> ni = nodeInfo;
     NS_TrustedNewXULElement(getter_AddRefs(mHScrollbarContent), ni.forget());
+#ifdef DEBUG
+    // Scrollbars can get restyled by theme changes.  Whether such a restyle
+    // will actually reconstruct them correctly if it involves a frame
+    // reconstruct... I don't know.  :(
+    mHScrollbarContent->SetProperty(nsGkAtoms::restylableAnonymousNode,
+                                    reinterpret_cast<void*>(true));
+#endif // DEBUG
+
     mHScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::orient,
                                 NS_LITERAL_STRING("horizontal"), false);
     mHScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::clickthrough,
                                 NS_LITERAL_STRING("always"), false);
     if (mIsRoot) {
       mHScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::root_,
                                   NS_LITERAL_STRING("true"), false);
     }
     if (!aElements.AppendElement(mHScrollbarContent))
       return NS_ERROR_OUT_OF_MEMORY;
   }
 
   if (canHaveVertical) {
     nsRefPtr<NodeInfo> ni = nodeInfo;
     NS_TrustedNewXULElement(getter_AddRefs(mVScrollbarContent), ni.forget());
+#ifdef DEBUG
+    // Scrollbars can get restyled by theme changes.  Whether such a restyle
+    // will actually reconstruct them correctly if it involves a frame
+    // reconstruct... I don't know.  :(
+    mVScrollbarContent->SetProperty(nsGkAtoms::restylableAnonymousNode,
+                                    reinterpret_cast<void*>(true));
+#endif // DEBUG
+
     mVScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::orient,
                                 NS_LITERAL_STRING("vertical"), false);
     mVScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::clickthrough,
                                 NS_LITERAL_STRING("always"), false);
     if (mIsRoot) {
       mVScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::root_,
                                   NS_LITERAL_STRING("true"), false);
     }