It's most likely using reVC, a reverse-engineering of the original binaries by decompilation, and then built for the web using emscripten, which does a fairly good job making OpenGL code work on WebGL.
My Tomb Raider web build I linked here elsewhere was done the same way (reversing by the amazing people in the TR1X project).
they are not, but then again so are many things. We choose what laws to enforce (see 18-20 year olds drinking, unmarried cohabitation, etc). just because it is not legal does not mean that law enforcement will care.
After 1 year of cohabitation in Canada, couples are federally common law spouses for tax purposes. Provincially it depends, but after 2 years in some provinces you're technically and automatically spouses for family law purposes, which gives someone the same legal rights as an explicitly married couple in terms of asset division and parental obligations following a breakup (which is outdated and insane in many cases outside of having children, but whatever).
Is it legal? Well, the reverse engineering typically is as long as you follow the rules, but hosting all the game assets on a public web server so you can play it probably isn't.
This isn't just reverse engineering, it's a decompiled source from the original binary.
how is this done ?? what engine is used ? it feels exactly like the original
also the whole website dos zone seems to have all these browser versions of half life etc ???
how are people making these things and how are they legal ?
so many questions