diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index 7a488379d848..4c29532ee216 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -2627,7 +2627,8 @@ nsCSSFrameConstructor::ConstructDocElementFrame(Element*                 aDocEle
     newFrame = frameItems.FirstChild();
     NS_ASSERTION(frameItems.OnlyChild(), "multiple root element frames");
   } else {
-    MOZ_ASSERT(display->mDisplay == StyleDisplay::Block,
+    MOZ_ASSERT(display->mDisplay == StyleDisplay::Block ||
+               display->mDisplay == StyleDisplay::FlowRoot,
                "Unhandled display type for root element");
     contentFrame = NS_NewBlockFormattingContext(mPresShell, styleContext);
     nsFrameItems frameItems;
@@ -4767,6 +4768,7 @@ nsCSSFrameConstructor::FindDisplayData(const nsStyleDisplay* aDisplay,
   static const FrameConstructionDataByDisplay sDisplayData[] = {
     FCDATA_FOR_DISPLAY(StyleDisplay::None, UNREACHABLE_FCDATA()),
     FCDATA_FOR_DISPLAY(StyleDisplay::Block, UNREACHABLE_FCDATA()),
+    FCDATA_FOR_DISPLAY(StyleDisplay::FlowRoot, UNREACHABLE_FCDATA()),
     // To keep the hash table small don't add inline frames (they're
     // typically things like FONT and B), because we can quickly
     // find them if we need to.
diff --git a/layout/generic/nsBlockFrame.cpp b/layout/generic/nsBlockFrame.cpp
index e6e3a2b290df..6907649af723 100644
--- a/layout/generic/nsBlockFrame.cpp
+++ b/layout/generic/nsBlockFrame.cpp
@@ -6907,6 +6907,7 @@ nsBlockFrame::Init(nsIContent*       aContent,
     AddStateBits(NS_BLOCK_NEEDS_BIDI_RESOLUTION);
   }
 
+  // A display:flow-root box establishes a block formatting context.
   // If a box has a different block flow direction than its containing block:
   // ...
   //   If the box is a block container, then it establishes a new block
@@ -6914,7 +6915,8 @@ nsBlockFrame::Init(nsIContent*       aContent,
   // (http://dev.w3.org/csswg/css-writing-modes/#block-flow)
   // If the box has contain: paint (or contain: strict), then it should also
   // establish a formatting context.
-  if ((GetParent() && StyleVisibility()->mWritingMode !=
+  if (StyleDisplay()->mDisplay == mozilla::StyleDisplay::FlowRoot ||
+      (GetParent() && StyleVisibility()->mWritingMode !=
                       GetParent()->StyleVisibility()->mWritingMode) ||
       StyleDisplay()->IsContainPaint()) {
     AddStateBits(NS_BLOCK_FLOAT_MGR | NS_BLOCK_MARGIN_ROOT);