Doppio (https://github.com/plasma-umass/doppio https://github.com/plasma-umass/doppio) is exactly this: a POSIX layer (for a single process), while Browsix is a

Doppio (https://github.com/plasma-umass/doppio https://github.com/plasma-umass/doppio) is exactly this: a POSIX layer (for a single process), while Browsix is a Unix layer. Both are JS only. Doppio implements a ton of libc functionality, enough to host a full JVM interpreter (see http://plasma-umass.github.io/doppio-demo/ http://plasma-umass.github.io/doppio-demo/). Doppio contains BrowserFS (available separately as https://github.com/jvilk/BrowserFS https://github.com/jvilk/BrowserFS), an in-browser file system that abstracts over local storage and lets you mount remote filesystems like Dropbox, Zip files, and more. Archive.org is using it for all of its DOS and Windows games.