diff --git a/CLOBBER b/CLOBBER
index ba7942e61..0dc1fdb0d 100644
--- a/CLOBBER
+++ b/CLOBBER
@@ -22,4 +22,4 @@
 # changes to stick? As of bug 928195, this shouldn't be necessary! Please
 # don't change CLOBBER for WebIDL changes any more.
 
-Clobber for NSS update
+Clobber for CSS flow-root implementation
diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index fc458b5eb..21068b673 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -2618,7 +2618,8 @@ 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;
@@ -4741,6 +4742,7 @@ 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.
@@ -4926,7 +4928,7 @@ nsCSSFrameConstructor::ConstructNonScrollableBlockWithConstructor(
        StyleDisplay::InlineBlock == aDisplay->mDisplay ||
        clipPaginatedOverflow) &&
       !aParentFrame->IsSVGText()) {
-    flags = NS_BLOCK_FLOAT_MGR | NS_BLOCK_MARGIN_ROOT;
+    flags = NS_BLOCK_FORMATTING_CONTEXT_STATE_BITS;
     if (clipPaginatedOverflow) {
       flags |= NS_BLOCK_CLIP_PAGINATED_OVERFLOW;
     }
@@ -5119,7 +5121,7 @@ nsCSSFrameConstructor::FlushAccumulatedBlock(nsFrameConstructorState& aState,
   // is not a suitable block.
   nsContainerFrame* blockFrame =
     NS_NewMathMLmathBlockFrame(mPresShell, blockContext);
-  blockFrame->AddStateBits(NS_BLOCK_FLOAT_MGR | NS_BLOCK_MARGIN_ROOT);
+  blockFrame->AddStateBits(NS_BLOCK_FORMATTING_CONTEXT_STATE_BITS);
 
   InitAndRestoreFrame(aState, aContent, aParentFrame, blockFrame);
   ReparentFrames(this, blockFrame, aBlockItems);
diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index 74dbd63e6..c8455c1c9 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -149,6 +149,7 @@ using namespace mozilla::gfx;
 #define GRID_ENABLED_PREF_NAME "layout.css.grid.enabled"
 #define GRID_TEMPLATE_SUBGRID_ENABLED_PREF_NAME "layout.css.grid-template-subgrid-value.enabled"
 #define WEBKIT_PREFIXES_ENABLED_PREF_NAME "layout.css.prefixes.webkit"
+#define DISPLAY_FLOW_ROOT_ENABLED_PREF_NAME "layout.css.display-flow-root.enabled"
 #define DISPLAY_CONTENTS_ENABLED_PREF_NAME "layout.css.display-contents.enabled"
 #define TEXT_ALIGN_UNSAFE_ENABLED_PREF_NAME "layout.css.text-align-unsafe-value.enabled"
 #define FLOAT_LOGICAL_VALUES_ENABLED_PREF_NAME "layout.css.float-logical-values.enabled"
@@ -314,6 +315,37 @@ WebkitPrefixEnabledPrefChangeCallback(const char* aPrefName, void* aClosure)
   }
 }
 
+// When the pref "layout.css.display-flow-root.enabled" changes, this function is
+// invoked to let us update kDisplayKTable, to selectively disable or restore
+// the entries for "flow-root" in that table.
+static void
+DisplayFlowRootEnabledPrefChangeCallback(const char* aPrefName, void* aClosure)
+{
+  NS_ASSERTION(strcmp(aPrefName, DISPLAY_FLOW_ROOT_ENABLED_PREF_NAME) == 0,
+               "Did you misspell " DISPLAY_FLOW_ROOT_ENABLED_PREF_NAME " ?");
+
+  static bool sIsDisplayFlowRootKeywordIndexInitialized;
+  static int32_t sIndexOfFlowRootInDisplayTable;
+  bool isDisplayFlowRootEnabled =
+    Preferences::GetBool(DISPLAY_FLOW_ROOT_ENABLED_PREF_NAME, false);
+
+  if (!sIsDisplayFlowRootKeywordIndexInitialized) {
+    // First run: find the position of "flow-root" in kDisplayKTable.
+    sIndexOfFlowRootInDisplayTable =
+      nsCSSProps::FindIndexOfKeyword(eCSSKeyword_flow_root,
+                                     nsCSSProps::kDisplayKTable);
+    sIsDisplayFlowRootKeywordIndexInitialized = true;
+  }
+
+  // OK -- now, stomp on or restore the "flow-root" entry in kDisplayKTable,
+  // depending on whether the pref is enabled vs. disabled.
+  if (sIndexOfFlowRootInDisplayTable >= 0) {
+    nsCSSProps::kDisplayKTable[sIndexOfFlowRootInDisplayTable].mKeyword =
+      isDisplayFlowRootEnabled ? eCSSKeyword_flow_root : eCSSKeyword_UNKNOWN;
+  }
+}
+
+
 // When the pref "layout.css.display-contents.enabled" changes, this function is
 // invoked to let us update kDisplayKTable, to selectively disable or restore
 // the entries for "contents" in that table.
@@ -7555,6 +7587,8 @@ static const PrefCallbacks kPrefCallbacks[] = {
     WebkitPrefixEnabledPrefChangeCallback },
   { TEXT_ALIGN_UNSAFE_ENABLED_PREF_NAME,
     TextAlignUnsafeEnabledPrefChangeCallback },
+  { DISPLAY_FLOW_ROOT_ENABLED_PREF_NAME,
+    DisplayFlowRootEnabledPrefChangeCallback },
   { DISPLAY_CONTENTS_ENABLED_PREF_NAME,
     DisplayContentsEnabledPrefChangeCallback },
   { FLOAT_LOGICAL_VALUES_ENABLED_PREF_NAME,
diff --git a/layout/forms/nsLegendFrame.cpp b/layout/forms/nsLegendFrame.cpp
index 95efc2c87..b985ed8fc 100644
--- a/layout/forms/nsLegendFrame.cpp
+++ b/layout/forms/nsLegendFrame.cpp
@@ -24,9 +24,7 @@ NS_NewLegendFrame(nsIPresShell* aPresShell, nsStyleContext* aContext)
 #endif
 
   nsIFrame* f = new (aPresShell) nsLegendFrame(aContext);
-  if (f) {
-    f->AddStateBits(NS_BLOCK_FLOAT_MGR | NS_BLOCK_MARGIN_ROOT);
-  }
+  f->AddStateBits(NS_BLOCK_FORMATTING_CONTEXT_STATE_BITS);
   return f;
 }
 
diff --git a/layout/generic/ReflowInput.cpp b/layout/generic/ReflowInput.cpp
index 9f7b4368c..077c68b5b 100644
--- a/layout/generic/ReflowInput.cpp
+++ b/layout/generic/ReflowInput.cpp
@@ -844,6 +844,7 @@ ReflowInput::InitFrameType(nsIAtom* aFrameType)
     case StyleDisplay::Flex:
     case StyleDisplay::WebkitBox:
     case StyleDisplay::Grid:
+    case StyleDisplay::FlowRoot:
     case StyleDisplay::RubyTextContainer:
       frameType = NS_CSS_FRAME_TYPE_BLOCK;
       break;
diff --git a/layout/generic/nsBlockFrame.cpp b/layout/generic/nsBlockFrame.cpp
index 4742db07e..152b3d06a 100644
--- a/layout/generic/nsBlockFrame.cpp
+++ b/layout/generic/nsBlockFrame.cpp
@@ -305,7 +305,7 @@ NS_NewBlockFormattingContext(nsIPresShell* aPresShell,
                              nsStyleContext* aStyleContext)
 {
   nsBlockFrame* blockFrame = NS_NewBlockFrame(aPresShell, aStyleContext);
-  blockFrame->AddStateBits(NS_BLOCK_FLOAT_MGR | NS_BLOCK_MARGIN_ROOT);
+  blockFrame->AddStateBits(NS_BLOCK_FORMATTING_CONTEXT_STATE_BITS);
   return blockFrame;
 }
 
@@ -6893,10 +6893,11 @@ 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);
+    AddStateBits(NS_BLOCK_FORMATTING_CONTEXT_STATE_BITS);
   }
 
   if ((GetStateBits() &
diff --git a/layout/generic/nsFrameStateBits.h b/layout/generic/nsFrameStateBits.h
index f8b1e541c..ba43e37d4 100644
--- a/layout/generic/nsFrameStateBits.h
+++ b/layout/generic/nsFrameStateBits.h
@@ -483,6 +483,9 @@ FRAME_STATE_BIT(Block, 22, NS_BLOCK_MARGIN_ROOT)
 // used to reserve space for the floated frames.
 FRAME_STATE_BIT(Block, 23, NS_BLOCK_FLOAT_MGR)
 
+// For setting the relevant bits on a block formatting context:
+#define NS_BLOCK_FORMATTING_CONTEXT_STATE_BITS (NS_BLOCK_FLOAT_MGR | NS_BLOCK_MARGIN_ROOT)
+
 FRAME_STATE_BIT(Block, 24, NS_BLOCK_HAS_LINE_CURSOR)
 
 FRAME_STATE_BIT(Block, 25, NS_BLOCK_HAS_OVERFLOW_LINES)
diff --git a/layout/generic/nsHTMLParts.h b/layout/generic/nsHTMLParts.h
index 243c432b2..97edaf75c 100644
--- a/layout/generic/nsHTMLParts.h
+++ b/layout/generic/nsHTMLParts.h
@@ -28,8 +28,7 @@ class nsTableColFrame;
 // These are all the block specific frame bits, they are copied from
 // the prev-in-flow to a newly created next-in-flow, except for the
 // NS_BLOCK_FLAGS_NON_INHERITED_MASK bits below.
-#define NS_BLOCK_FLAGS_MASK (NS_BLOCK_MARGIN_ROOT              | \
-                             NS_BLOCK_FLOAT_MGR                | \
+#define NS_BLOCK_FLAGS_MASK (NS_BLOCK_FORMATTING_CONTEXT_STATE_BITS | \
                              NS_BLOCK_CLIP_PAGINATED_OVERFLOW  | \
                              NS_BLOCK_HAS_FIRST_LETTER_STYLE   | \
                              NS_BLOCK_FRAME_HAS_OUTSIDE_BULLET | \
diff --git a/layout/style/nsCSSKeywordList.h b/layout/style/nsCSSKeywordList.h
index 34a46ffce..9045da9ff 100644
--- a/layout/style/nsCSSKeywordList.h
+++ b/layout/style/nsCSSKeywordList.h
@@ -285,6 +285,7 @@ CSS_KEY(flex, flex)
 CSS_KEY(flex-end, flex_end)
 CSS_KEY(flex-start, flex_start)
 CSS_KEY(flip, flip)
+CSS_KEY(flow-root, flow_root)
 CSS_KEY(forwards, forwards)
 CSS_KEY(fraktur, fraktur)
 CSS_KEY(from-image, from_image)
diff --git a/layout/style/nsCSSProps.cpp b/layout/style/nsCSSProps.cpp
index ac2978c27..24c97cf33 100644
--- a/layout/style/nsCSSProps.cpp
+++ b/layout/style/nsCSSProps.cpp
@@ -1345,6 +1345,9 @@ KTableEntry nsCSSProps::kDisplayKTable[] = {
   // The next entry is controlled by the layout.css.display-contents.enabled
   // pref.
   { eCSSKeyword_contents,            StyleDisplay::Contents },
+  // The next entry is controlled by the layout.css.display-flow-root.enabled 
+  // pref.
+  { eCSSKeyword_flow_root,           StyleDisplay::FlowRoot },
   { eCSSKeyword_UNKNOWN,             -1 }
 };
 
diff --git a/layout/style/nsRuleNode.cpp b/layout/style/nsRuleNode.cpp
index 1a451a2ef..036d97f86 100644
--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -250,6 +250,7 @@ nsRuleNode::EnsureBlockDisplay(StyleDisplay& display,
   case StyleDisplay::Flex:
   case StyleDisplay::WebkitBox:
   case StyleDisplay::Grid:
+  case StyleDisplay::FlowRoot:
     // do not muck with these at all - already blocks
     // This is equivalent to nsStyleDisplay::IsBlockOutside.  (XXX Maybe we
     // should just call that?)
@@ -293,6 +294,7 @@ nsRuleNode::EnsureInlineDisplay(StyleDisplay& display)
   // see if the display value is already inline
   switch (display) {
     case StyleDisplay::Block:
+    case StyleDisplay::FlowRoot:
       display = StyleDisplay::InlineBlock;
       break;
     case StyleDisplay::Table:
diff --git a/layout/style/nsStyleConsts.h b/layout/style/nsStyleConsts.h
index 6d207aec9..f54387aa8 100644
--- a/layout/style/nsStyleConsts.h
+++ b/layout/style/nsStyleConsts.h
@@ -524,6 +524,7 @@ enum class FillMode : uint32_t;
 enum class StyleDisplay : uint8_t {
   None = 0,
   Block,
+  FlowRoot,
   Inline,
   InlineBlock,
   ListItem,
diff --git a/layout/style/nsStyleStruct.h b/layout/style/nsStyleStruct.h
index 4bda817dd..f49cdc43e 100644
--- a/layout/style/nsStyleStruct.h
+++ b/layout/style/nsStyleStruct.h
@@ -2890,7 +2890,8 @@ struct MOZ_NEEDS_MEMMOVABLE_MEMBERS nsStyleDisplay
     return mozilla::StyleDisplay::Block == mDisplay ||
            mozilla::StyleDisplay::ListItem == mDisplay ||
            mozilla::StyleDisplay::InlineBlock == mDisplay ||
-           mozilla::StyleDisplay::TableCaption == mDisplay;
+           mozilla::StyleDisplay::TableCaption == mDisplay ||
+           mozilla::StyleDisplay::FlowRoot == mDisplay;
     // Should TABLE_CELL be included here?  They have
     // block frames nested inside of them.
     // (But please audit all callers before changing.)
@@ -2902,7 +2903,8 @@ struct MOZ_NEEDS_MEMMOVABLE_MEMBERS nsStyleDisplay
            mozilla::StyleDisplay::WebkitBox == mDisplay ||
            mozilla::StyleDisplay::Grid == mDisplay ||
            mozilla::StyleDisplay::ListItem == mDisplay ||
-           mozilla::StyleDisplay::Table == mDisplay;
+           mozilla::StyleDisplay::Table == mDisplay ||
+           mozilla::StyleDisplay::FlowRoot == mDisplay;
   }
 
   static bool IsDisplayTypeInlineOutside(mozilla::StyleDisplay aDisplay) {
diff --git a/layout/style/test/property_database.js b/layout/style/test/property_database.js
index c75f7b498..2d6352148 100644
--- a/layout/style/test/property_database.js
+++ b/layout/style/test/property_database.js
@@ -7892,6 +7892,10 @@ if (IsCSSPropertyPrefEnabled("layout.css.background-clip-text.enabled")) {
   );
 }
 
+if (IsCSSPropertyPrefEnabled("layout.css.display-flow-root.enabled")) {
+  gCSSProperties["display"].other_values.push("flow-root");
+}
+
 // Copy aliased properties' fields from their alias targets.
 for (var prop in gCSSProperties) {
   var entry = gCSSProperties[prop];
diff --git a/layout/xul/nsXULLabelFrame.cpp b/layout/xul/nsXULLabelFrame.cpp
index 22b875461..41e7e4d76 100644
--- a/layout/xul/nsXULLabelFrame.cpp
+++ b/layout/xul/nsXULLabelFrame.cpp
@@ -16,9 +16,7 @@ nsIFrame*
 NS_NewXULLabelFrame(nsIPresShell* aPresShell, nsStyleContext* aContext)
 {
   nsXULLabelFrame* it = new (aPresShell) nsXULLabelFrame(aContext);
-
-  it->AddStateBits(NS_BLOCK_FLOAT_MGR | NS_BLOCK_MARGIN_ROOT);
-
+  it->AddStateBits(NS_BLOCK_FORMATTING_CONTEXT_STATE_BITS);
   return it;
 }
 
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index cf8c16a62..35dee094f 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -2607,6 +2607,9 @@ pref("layout.css.grid-template-subgrid-value.enabled", false);
 // Is support for CSS contain enabled?
 pref("layout.css.contain.enabled", false);
 
+// Is support for CSS display:flow-root enabled?
+pref("layout.css.display-flow-root.enabled", true);
+
 // Is support for CSS display:contents enabled?
 pref("layout.css.display-contents.enabled", true);