On a technical level it's actually not that impressive. Mozilla got BananaBread/Sauerbraten running with WebGL on netbooks over a decade ago, with much more advanced graphics than Xash3D:
The Half Life "app" above actually just wraps an existing Emscripten port in an IFrame. Puter does provide an abstracted filesystem (and other "OS" features), but I don't think the app above uses that, so it's no different than if you went to `data:text/html,IntralexicalOS!<br><iframe src="https://pixelsuft.github.io/hl/" style="width:50%;height:50%">`.
But actually I think putting together such a simple technical concept that works this well is the impressive part, that hasn't been done before. You can now take any existing web build of any app, and by adding a couple calls to `puter.fs.write()`, deploy it to a familiar virtual workspace where you get multitasking, cloud storage, and interoperation with other apps, for free.
I've seen many 'desktop in browser' type demos over the years, but never anything like this. This is impressive on so many levels.