--- a/layout/base/FrameLayerBuilder.cpp
+++ b/layout/base/FrameLayerBuilder.cpp
@@ -1556,16 +1556,21 @@ struct CSSMaskLayerUserData : public Lay
     }
 
     return true;
   }
 
   nsStyleImageLayers mImageLayer;
 };
 
+/*
+ * A helper object to create a draw target for mask painting and create a
+ * image container to hold the drawing result. The caller can then bind this
+ * image container with a image mask layer via ImageLayer::SetContainer.
+ */
 class MaskImageData
 {
 public:
   MaskImageData(const gfx::IntSize& aSize, LayerManager* aLayerManager)
     : mTextureClientLocked(false)
     , mSize(aSize)
     , mLayerManager(aLayerManager)
   {