I think it's PFM. I'm not seeing anything in the simulator that lets you add a CRT type of display. [Edit] Yeah, this is the case. If you view the page source,

I think it's PFM. I'm not seeing anything in the simulator that lets you add a CRT type of display. [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.