diff -r 000000000000 -r 6474c204b198 gfx/2d/BaseMargin.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gfx/2d/BaseMargin.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,77 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 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/. */
+
+#ifndef MOZILLA_GFX_BASEMARGIN_H_
+#define MOZILLA_GFX_BASEMARGIN_H_
+
+#include "Types.h"
+
+namespace mozilla {
+namespace gfx {
+
+/**
+ * Do not use this class directly. Subclass it, pass that subclass as the
+ * Sub parameter, and only use that subclass.
+ */
+template <class T, class Sub>
+struct BaseMargin {
+  typedef mozilla::css::Side SideT;
+
+  // Do not change the layout of these members; the Side() methods below
+  // depend on this order.
+  T top, right, bottom, left;
+
+  // Constructors
+  BaseMargin() : top(0), right(0), bottom(0), left(0) {}
+  BaseMargin(T aTop, T aRight, T aBottom, T aLeft) :
+      top(aTop), right(aRight), bottom(aBottom), left(aLeft) {}
+
+  void SizeTo(T aTop, T aRight, T aBottom, T aLeft)
+  {
+    top = aTop; right = aRight; bottom = aBottom; left = aLeft;
+  }
+
+  T LeftRight() const { return left + right; }
+  T TopBottom() const { return top + bottom; }
+
+  T& Side(SideT aSide) {
+    // This is ugly!
+    return *(&top + T(aSide));
+  }
+  T Side(SideT aSide) const {
+    // This is ugly!
+    return *(&top + T(aSide));
+  }
+
+  // Overloaded operators. Note that '=' isn't defined so we'll get the
+  // compiler generated default assignment operator
+  bool operator==(const Sub& aMargin) const {
+    return top == aMargin.top && right == aMargin.right &&
+           bottom == aMargin.bottom && left == aMargin.left;
+  }
+  bool operator!=(const Sub& aMargin) const {
+    return !(*this == aMargin);
+  }
+  Sub operator+(const Sub& aMargin) const {
+    return Sub(top + aMargin.top, right + aMargin.right,
+               bottom + aMargin.bottom, left + aMargin.left);
+  }
+  Sub operator-(const Sub& aMargin) const {
+    return Sub(top - aMargin.top, right - aMargin.right,
+               bottom - aMargin.bottom, left - aMargin.left);
+  }
+  Sub& operator+=(const Sub& aMargin) {
+    top += aMargin.top;
+    right += aMargin.right;
+    bottom += aMargin.bottom;
+    left += aMargin.left;
+    return *static_cast<Sub*>(this);
+  }
+};
+
+}
+}
+
+#endif /* MOZILLA_GFX_BASEMARGIN_H_ */