JavaFX JavaDoc is a very good source for any extra information you want to know about the JavaFX API. It's usually accompanied with examples, and can be quite thorough.

JavaDoc can be found online: https://docs.oracle.com/javase/10/docs/api/overview-summary.html. Also, it's included in the JDK distribution and most IDEs will show it in code completion, or by a shortcut.

Besides JavaDoc, there are great reference guides to FXML and CSS in the official documentation:

• FXML: https://docs.oracle.com/javase/10/docs/api/javafx/fxml/doc-files/introduction_to_fxml.html
• CSS: https://docs.oracle.com/javase/10/docs/api/javafx/scene/doc-files/cssref.html

The JavaFX articles set, unfortunately, hasn't been updated much since Java8, but still, it is a good source of information, https://docs.oracle.com/javase/8/javase-clienttechnologies.htm.