Kinda. Not quite. This is an application¹ that registers a new protocol (in my code that’s syrinscape: but that’s my own choice).
It doesn’t change anything about the behavior of normal links at all. If you want to go to a user’s profile or anything, you’re talking to https://www.syrinscape.com/online/… and that happens in the browser.
But the DOE links are changed by the GM to point at, say syrinscape:elements/20/play/ for a fireball effect. DOE tells the OS to ‘open’ that, the OS calls the application, the application builds a request to the Syrinscape API and throws the result away, terminating afterwards (no windows pop up). If you keep https links in FG/DOE nothing changes and you get browser popups.
So if you bundle a small tool like this / register a protocol handler during installation, you’d support DOE (and potentially other stuff that works similarly) without changing your current behavior/player at all, for all I can tell.
①: Nitpicking, but applets remind me of a dead Java thing that’s unrelated