> JavaFX uses custom XML which make no sense to be honest when you want to do nice design.

I don't see why the custom XML should be a problem. Definitely you are not supposed to edit the .fxml files by hand, especially when you have apps like Scene Builder that allow you to design everything from a GUI.

> And it requires custom CSS too.

That's true only for a few JavaFX-specific properties. The CSS syntax doesn't change very much. Also, it's not a surprising feature: QT, the C++ GUI framework, also uses custom CSS.