This is made by StackBlitz (not really, see reply to this comment https://news.ycombinator.com/item?id=29812650 ) who make a Cloud IDE that runs in the browser instead of a server. As for the why of that see https://blog.stackblitz.com/posts/introducing-webcontainers/ "What about Code Spaces/Sandbox/Repls/...? Legacy online IDEs run your entire dev environment on a remote server and stream the results back across the internet to your browser. The problem with this approach is that it yields few security benefits and provides a worse experience than your local machine in nearly every way: it takes minutes to spin up containers, is prone to network latency, cannot work offline, often results in network timeouts, debugging frozen/broken containers is nearly impossible, and hitting refresh just reconnects you to the broken container again."