In a way it really is, in part because HTML + CSS is the back door "universal native GUI framework" that various projects have been chasing for years. Because browsers and GUI elements in them are so important to modern computer use, every OS vendor has a vested interest in ensuring the default look and behavior of elements in their browsers are native (or as native as possible). As a result, if you can render your UI in a browser (or in a browser frame/view) chances are you can get a native looking and native feeling UI for your application with a well understood and robust piece of technology that will retain that look and feel even as the underlying OS changes around it.