Margins that start at fragmentainer boundaries should be collapsed away.

third_party/WebKit/Source/core/layout/LayoutBlockFlow.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2003-2013 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2010. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ skipping 760 matching lines @@
   virtual ETextAlign textAlignmentForLine(bool endsWithSoftBreak) const;
 
  private:
   LayoutUnit collapsedMarginBefore() const final {
     return maxPositiveMarginBefore() - maxNegativeMarginBefore();
   }
   LayoutUnit collapsedMarginAfter() const final {
     return maxPositiveMarginAfter() - maxNegativeMarginAfter();
   }
 
+  LayoutUnit adjustedMarginBeforeForPagination(
+      const LayoutBox&,
+      LayoutUnit logicalTopMarginEdge,
+      LayoutUnit logicalTopBorderEdge,
+      const BlockChildrenLayoutInfo&) const;
+
   LayoutUnit collapseMargins(LayoutBox& child,
-                             MarginInfo&,
+                             BlockChildrenLayoutInfo&,
                              bool childIsSelfCollapsing,
                              bool childDiscardMarginBefore,
                              bool childDiscardMarginAfter);
   LayoutUnit clearFloatsIfNeeded(LayoutBox& child,
                                  MarginInfo&,
                                  LayoutUnit oldTopPosMargin,
                                  LayoutUnit oldTopNegMargin,
                                  LayoutUnit yPos,
                                  bool childIsSelfCollapsing,
                                  bool childDiscardMargin);
@@ skipping 147 matching lines @@
   void positionDialog();
 
   // END METHODS DEFINED IN LayoutBlockFlowLine
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutBlockFlow, isLayoutBlockFlow());
 
 }  // namespace blink
 
 #endif  // LayoutBlockFlow_h