In addition the widgets as such might be rendered differently for different platforms (in Smalltalk in one of the GUI frameworks, an example was Squeak Morphic and MVC ) or outside (e.g. Kivy).
@Hilaire, what do you think is outstanding of special about Kivy?
I think to illustrate this it might be a nice experiment to develop a 'hello world' equivalent for GUI construction and have it rendered in as many user interface languages as possible (Kivy, various types of Morphic, Bloc etc., but as PPTX and ODP)
Theoretically speaking this is already possible with Spec, which is backend-independent GUI framework connected to backends (Morphic) via adapters. It would be an interesting proof-of-concept to implement other backends such as HTML without touching Spec. Peter