I love that luminaries from Oxford, no less than the Particle Physics department of Oxford university use Duke Nukem 3D as their example how how much the emulator is kicking ass and chewing bubblegum.
Author here: The sourceforge website hasn't been updated in years, and yes it used to launch demos in the browser, but most people don't have Java in the browser these days. The github page is more recent.
Clicking any of the screenshots just goes to the oracle site, I assume originally this would have launched JPC with the relevant image.
There is a messaging saying "You need to update your browser's Java" which most likely means it is either disabled in the browser or just not installed.