I'm curious about the tech in some of these. Probably a mix of straight up HTML reinterpretation, probably some web assembly, feels like at least one or two are running a full win9x dist in either docker or something...
There's some frameworks that are available for old Windows desktop apps to run in the browser. It's main use is for old ERP systems that were originally written for on-premise to be deployed in the cloud and browser based without needing to essentially rewrite everything.
The one tech stack I know of that does this is IBM Websphere for Java apps (https://en.wikipedia.org/wiki/IBM_WebSphere). One that isn't as smooth as the site in OP is Microsoft's Remote Desktop Service.
