I tried to test a simple JavaFX program using Java with Swing and regular Java, but both attempts could not import the javafx package.
Has anyone been able to get JavaFX to work in repl.it?
As of 2020-12-28 it is feasible but it is not pretty. You can do it if you upload the whole OpenJFX library in your Repl.it. This can be difficult because the library is about 100Mb. And then you can override how the "Run" button works by default using a ".replit" file and a Bash script.
I made an example here: https://repl.it/@mlavallee/JavaFxTest (sorry comments are in French in the code)