What we know
The Xash3D engine (an open reimplementation of GoldSrc) compiled for the browser. It plays Half-Life and Counter-Strike 1.6 from game files you supply.
Links
Source and tools
Deep dive in progress
This page is a short entry. The full write-up (original game, who built the port, how to play, creators and sources) comes next.
You will need game files from a copy you own. Unofficial fan project: it can be removed without notice. Not legal advice. Never run a launcher or download from a page you do not trust.