I hope this can be made to work securely. The main thing holding back web-based IDEs is file system access. It wouldn't be horribly difficult to port e.g. Emacs

I hope this can be made to work securely. The main thing holding back web-based IDEs is file system access. It wouldn't be horribly difficult to port e.g. Emacs to Javascript (or more likely WebAssembly) if file access was available. I know this sounds ridiculous, but it would be doable.