Krunker

Krunker is the proof that a browser can host a competent competitive shooter. It loads in seconds, runs at high frame rates on modest hardware, and its movement, slide-hopping in particular, has enough depth that the skill gap between a new player and a practised one is immediately obvious.

There are a dozen or so classes with distinct weapons and speeds, a large library of community maps, and custom lobbies you can share with a link. It will not replace Counter-Strike, but as the game you can start playing on a work laptop in under thirty seconds it has no real competition.