I know this is a late response, but for anyone curious you would need to use an official Oracle or Sun JDK/JRE from Java 8 or older. OpenJDK doesn't include support for applets.

You also need a browser that has NPAPI. IE 11 was the most modern browser I am aware of that still supported applets.

The old GUI framework mentioned in the GP might have been Swing. It is still included in most JDKs and allows for cross platform desktop GUI application development with no other dependencies outside the JDK. Finding documentation on how to do GUIs in Swing is getting increasingly difficult though.

IIRC Oracle makes it really hard to find the download. Admittedly we didn't spend more than an hour or two messing around with all of it and getting the applets to run wasn't a high priority or anything.