It looks to be an implementation of "browser in a browser". Originally the project seemed to handle usually unsupported apps, like IMAP clients in the browser. I assume they have a server running, and JS libraries that act as a shim for IMAP et al control via websockets. Also appears to create usable web-based proxies for modern security-controlled websites which would break on legacy web-proxy methods.

Ah, it looks like they've MuleSoft'd ViewFinderJS: https://news.ycombinator.com/item?id=28015601

Add the most requested features to a commercial offering.