diff --git a/layout/base/LayoutConstants.h b/layout/base/LayoutConstants.h
index bf86171d71bc..ac3714cf9a81 100644
--- a/layout/base/LayoutConstants.h
+++ b/layout/base/LayoutConstants.h
@@ -28,4 +28,10 @@
 
 #define NS_INTRINSIC_ISIZE_UNKNOWN nscoord_MIN
 
+// The fallback size of width is 300px and the aspect-ratio is 2:1, based on the
+// spec definition in CSS2 section 10.3.2:
+// https://drafts.csswg.org/css2/visudet.html#inline-replaced-width
+#define REPLACED_ELEM_FALLBACK_PX_WIDTH 300
+#define REPLACED_ELEM_FALLBACK_PX_HEIGHT 150
+
 #endif  // LayoutConstants_h___
diff --git a/layout/generic/nsSubDocumentFrame.cpp b/layout/generic/nsSubDocumentFrame.cpp
index 15108afbb4d0..b18e8c42ef42 100644
--- a/layout/generic/nsSubDocumentFrame.cpp
+++ b/layout/generic/nsSubDocumentFrame.cpp
@@ -624,9 +624,11 @@ IntrinsicSize nsSubDocumentFrame::GetIntrinsicSize() {
     return {};  // XUL <iframe> and <browser> have no useful intrinsic size
   }
 
-  // We must be an HTML <iframe>.  Default to size of 300px x 150px, for IE
-  // compat (and per CSS2.1 draft)
-  return IntrinsicSize(CSSPixel::ToAppUnits(300), CSSPixel::ToAppUnits(150));
+  // We must be an HTML <iframe>.  Default to size of
+  // REPLACED_ELEM_FALLBACK_PX_WIDTH x REPLACED_ELEM_FALLBACK_PX_HEIGHT (i.e.
+  // 300px x 150px), for IE compat (and per CSS2.1 draft)
+  return IntrinsicSize(CSSPixel::ToAppUnits(REPLACED_ELEM_FALLBACK_PX_WIDTH),
+                       CSSPixel::ToAppUnits(REPLACED_ELEM_FALLBACK_PX_HEIGHT));
 }
 
 /* virtual */
diff --git a/layout/generic/nsVideoFrame.cpp b/layout/generic/nsVideoFrame.cpp
index 5b8f69bf2931..20ba701cd61d 100644
--- a/layout/generic/nsVideoFrame.cpp
+++ b/layout/generic/nsVideoFrame.cpp
@@ -671,8 +671,8 @@ nsSize nsVideoFrame::GetVideoIntrinsicSize(gfxContext* aRenderingContext) {
     return nsSize(0, 0);
   }
 
-  // Defaulting size to 300x150 if no size given.
-  nsIntSize size(300, 150);
+  nsIntSize size(REPLACED_ELEM_FALLBACK_PX_WIDTH,
+                 REPLACED_ELEM_FALLBACK_PX_HEIGHT);
 
   HTMLVideoElement* element = static_cast<HTMLVideoElement*>(GetContent());
   if (NS_FAILED(element->GetVideoSize(&size)) && ShouldDisplayPoster()) {