In Android there is a concept where the developer can specify which version the developer is targeting. This makes it possible to change certain things when new versions of the platform comes out and doesn't freeze defaults and behaviour for all eternity.

JavaFX should have this same option. If the developer explicitly say that "I have tested this on JavaFX 8" then defaults new to JavaFX 8 can be used. Otherwise the old defaults are used.

There are many ways to do this but it needs to be implemented asap to support evolving the API.