diff --git a/dom/base/nsGkAtomList.h b/dom/base/nsGkAtomList.h
index d303ef8184bc..008825a2cc95 100644
--- a/dom/base/nsGkAtomList.h
+++ b/dom/base/nsGkAtomList.h
@@ -126,6 +126,7 @@ GK_ATOM(autoplay, "autoplay")
 GK_ATOM(autorepeatbutton, "autorepeatbutton")
 GK_ATOM(axis, "axis")
 GK_ATOM(b, "b")
+GK_ATOM(backdropFrame, "BackdropFrame")
 GK_ATOM(background, "background")
 GK_ATOM(base, "base")
 GK_ATOM(basefont, "basefont")
diff --git a/layout/generic/moz.build b/layout/generic/moz.build
index a0e60728db8f..f971b6600865 100644
--- a/layout/generic/moz.build
+++ b/layout/generic/moz.build
@@ -115,6 +115,7 @@ UNIFIED_SOURCES += [
     'FrameChildList.cpp',
     'MathMLTextRunFactory.cpp',
     'nsAbsoluteContainingBlock.cpp',
+    'nsBackdropFrame.cpp',
     'nsBlockFrame.cpp',
     'nsBlockReflowContext.cpp',
     'nsBlockReflowState.cpp',
diff --git a/layout/generic/nsBackdropFrame.cpp b/layout/generic/nsBackdropFrame.cpp
new file mode 100644
index 000000000000..135d8e3f482b
--- /dev/null
+++ b/layout/generic/nsBackdropFrame.cpp
@@ -0,0 +1,92 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+// vim:cindent:ts=2:et:sw=2:
+/* 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/. */
+
+/* rendering object for CSS "::backdrop" */
+
+#include "nsBackdropFrame.h"
+
+using namespace mozilla;
+
+NS_IMPL_FRAMEARENA_HELPERS(nsBackdropFrame)
+
+/* virtual */ nsIAtom*
+nsBackdropFrame::GetType() const
+{
+  return nsGkAtoms::backdropFrame;
+}
+
+#ifdef DEBUG_FRAME_DUMP
+nsresult
+nsBackdropFrame::GetFrameName(nsAString& aResult) const
+{
+  return MakeFrameName(NS_LITERAL_STRING("Backdrop"), aResult);
+}
+#endif
+
+/* virtual */ nsStyleContext*
+nsBackdropFrame::GetParentStyleContext(nsIFrame** aProviderFrame) const
+{
+  // Style context of backdrop pseudo-element does not inherit from
+  // any element, per the Fullscreen API spec.
+  *aProviderFrame = nullptr;
+  return nullptr;
+}
+
+/* virtual */ void
+nsBackdropFrame::BuildDisplayList(nsDisplayListBuilder* aBuilder,
+                                  const nsRect& aDirtyRect,
+                                  const nsDisplayListSet& aLists)
+{
+  DO_GLOBAL_REFLOW_COUNT_DSP("nsBackdropFrame");
+  // We want this frame to always be there even if its display value is
+  // none or contents so that we can respond to style change on it. To
+  // support those values, we skip painting ourselves in those cases.
+  auto display = StyleDisplay()->mDisplay;
+  if (display != NS_STYLE_DISPLAY_NONE &&
+      display != NS_STYLE_DISPLAY_CONTENTS) {
+    DisplayBorderBackgroundOutline(aBuilder, aLists);
+  }
+}
+
+/* virtual */ LogicalSize
+nsBackdropFrame::ComputeAutoSize(nsRenderingContext *aRenderingContext,
+                                 WritingMode aWM,
+                                 const LogicalSize& aCBSize,
+                                 nscoord aAvailableISize,
+                                 const LogicalSize& aMargin,
+                                 const LogicalSize& aBorder,
+                                 const LogicalSize& aPadding,
+                                 bool aShrinkWrap)
+{
+  // Note that this frame is a child of the viewport frame.
+  LogicalSize result(aWM, 0xdeadbeef, NS_UNCONSTRAINEDSIZE);
+  if (aShrinkWrap) {
+    result.ISize(aWM) = 0;
+  } else {
+    result.ISize(aWM) = aAvailableISize - aMargin.ISize(aWM) -
+                        aBorder.ISize(aWM) - aPadding.ISize(aWM);
+  }
+  return result;
+}
+
+/* virtual */ void
+nsBackdropFrame::Reflow(nsPresContext* aPresContext,
+                        nsHTMLReflowMetrics& aDesiredSize,
+                        const nsHTMLReflowState& aReflowState,
+                        nsReflowStatus& aStatus)
+{
+  MarkInReflow();
+  DO_GLOBAL_REFLOW_COUNT("nsBackdropFrame");
+  DISPLAY_REFLOW(aPresContext, this, aReflowState, aDesiredSize, aStatus);
+
+  // Note that this frame is a child of the viewport frame.
+  WritingMode wm = aReflowState.GetWritingMode();
+  LogicalMargin borderPadding = aReflowState.ComputedLogicalBorderPadding();
+  nscoord isize = aReflowState.ComputedISize() + borderPadding.IStartEnd(wm);
+  nscoord bsize = aReflowState.ComputedBSize() + borderPadding.BStartEnd(wm);
+  aDesiredSize.SetSize(wm, LogicalSize(wm, isize, bsize));
+  aStatus = NS_FRAME_COMPLETE;
+}
diff --git a/layout/generic/nsBackdropFrame.h b/layout/generic/nsBackdropFrame.h
new file mode 100644
index 000000000000..1babc8dd9180
--- /dev/null
+++ b/layout/generic/nsBackdropFrame.h
@@ -0,0 +1,46 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+// vim:cindent:ts=2:et:sw=2:
+/* 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/. */
+
+/* rendering object for CSS "::backdrop" */
+
+#ifndef nsBackdropFrame_h___
+#define nsBackdropFrame_h___
+
+#include "nsFrame.h"
+
+class nsBackdropFrame final : public nsFrame
+{
+public:
+  NS_DECL_FRAMEARENA_HELPERS
+
+  explicit nsBackdropFrame(nsStyleContext* aContext)
+    : nsFrame(aContext) { }
+
+  // nsIFrame overrides
+  virtual nsIAtom* GetType() const override;
+#ifdef DEBUG_FRAME_DUMP
+  virtual nsresult GetFrameName(nsAString& aResult) const override;
+#endif
+  virtual nsStyleContext*
+    GetParentStyleContext(nsIFrame** aProviderFrame) const override;
+  virtual void BuildDisplayList(nsDisplayListBuilder* aBuilder,
+                                const nsRect& aDirtyRect,
+                                const nsDisplayListSet& aLists) override;
+  virtual LogicalSize ComputeAutoSize(nsRenderingContext *aRenderingContext,
+                                      WritingMode aWM,
+                                      const LogicalSize& aCBSize,
+                                      nscoord aAvailableISize,
+                                      const LogicalSize& aMargin,
+                                      const LogicalSize& aBorder,
+                                      const LogicalSize& aPadding,
+                                      bool aShrinkWrap) override;
+  virtual void Reflow(nsPresContext* aPresContext,
+                      nsHTMLReflowMetrics& aDesiredSize,
+                      const nsHTMLReflowState& aReflowState,
+                      nsReflowStatus& aStatus) override;
+};
+
+#endif // nsBackdropFrame_h___
diff --git a/layout/generic/nsFrameIdList.h b/layout/generic/nsFrameIdList.h
index 10d806d976f4..955359076205 100644
--- a/layout/generic/nsFrameIdList.h
+++ b/layout/generic/nsFrameIdList.h
@@ -5,6 +5,7 @@
 FRAME_ID(BRFrame)
 FRAME_ID(nsAutoRepeatBoxFrame)
 FRAME_ID(nsBCTableCellFrame)
+FRAME_ID(nsBackdropFrame)
 FRAME_ID(nsBlockFrame)
 FRAME_ID(nsBox)
 FRAME_ID(nsBoxFrame)