--- a/gfx/2d/BaseMargin.h
+++ b/gfx/2d/BaseMargin.h
@@ -1,16 +1,18 @@
 /* -*- 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 <ostream>
+
 #include "Types.h"
 
 namespace mozilla {
 
 /**
  * Sides represents a set of physical sides.
  */
 struct Sides final {
@@ -131,14 +133,20 @@ struct BaseMargin {
   }
   Sub& operator+=(const Sub& aMargin) {
     top += aMargin.top;
     right += aMargin.right;
     bottom += aMargin.bottom;
     left += aMargin.left;
     return *static_cast<Sub*>(this);
   }
+
+  friend std::ostream& operator<<(std::ostream& aStream,
+      const BaseMargin& aMargin) {
+    return aStream << '(' << aMargin.top << ',' << aMargin.right << ','
+                  << aMargin.bottom << ',' << aMargin.left << ')';
+  }
 };
 
 } // namespace gfx
 } // namespace mozilla
 
 #endif /* MOZILLA_GFX_BASEMARGIN_H_ */