Though I don't really understand how an image is produced on a CRT. In this circuit (https://www.falstad.com/pong/net.html), what is "NET" actually connected to?
I don't what monitor type was assumed in this design but a similar approach could create a classic NTSC signal into a TV. Here is a solution with a micro controller http://eyetap.org/ece385/lab5.htm
Idk what "NET" is, though.
[Edit] Yeah, this is the case. If you view the page source, there's some custom JS drawing "NET" to a canvas. It's hard coded to pull from specific wires when they cross certain voltage thresholds in the simulation.
[Edit2] The implementation is also very buggy. The CRT simulation is just looping through X&Y completely decoupled from the circuit sim. Simple operations like stopping and running the sim while changing simulation speed will cause it to get off.
You can check this by deleting the input to net and then connecting a wire from some slow moving signal to net (so you can see how it responds in the image).
The CRT itself isn't actually modeled of course -- it is external hardware.