Ah, you must mean the one embedded in this article:

https://www.damninteresting.com/the-martian-express/

Originally, in 2008, it was 100% Javascript, but a few years later I updated it to use CSS transitions for a much smoother animation.

That cycler simulator actually led to a very cool life experience. An administrator at NASA contacted me wanting to use it for a presentation. Of course I said yes! Then he invited me to be his guest to watch an early test of the SLS rocket that was happening about 90 minutes from my home. Of course I said yes!

I got to meet a bunch of astronauts, see a big rocket engine fire horizontally in the desert, and got a NASA SLS pin. The fellow ended up not using the animation, as that part of his talk got edited out, but nevertheless niftiness.