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BorderBrush, BorderThickness are ignored on Viewbox.
Viewbox inherits from Decorator, which doesn't exist in Silverlight and because Silverlight doesn't have [Add|Remove][Logical|Visual]Child, we can't inherit from FrameworkElement. The only way to render Viewbox.Child is to have Viewbox inherit from Control, which has a Template, where we can use a ContentPresenter to render Viewbox.Child. By inheriting from Control, Viewbox unfortunately inherits all Control properties/events that don't make sense to it. We compensate that by sealing Viewbox, disregard all irrelevant Control properites like BorderBrush and BorderThickness etc.
We do have to throw if user tries to set Template, since Viewbox depends on default template to work.