https://play-cs.com/en/servers

Been quite surprised to find fully functional CS 1.6 in a browser, but that's it. Heard that the latency is worse compared to the game client, haven't checked that myself because I don't want to bother buying or pirating the original version. Main use case: playing at work when you can't install random binaries but can visit websites (this use case isn't mine but I've been asking players there and they responded in this way).

What is the technology behind this? Did they just port over the game using pure JS?

https://github.com/FWGS/cs16-client (but this is closed source now, I don't know exactly how they use it, maybe there is some different fork of `cs16-client`)

These two are what their own references point to when you load the game.