I've been hacking on my own task manager on-and-off for years (https://the.do.zone). It has no database; the primary store is the browser, and you can optionally sync to local file system (if using chrome), dropbox, or google drive (less optimal for my needs, because their js oauth implementation is very conservative about token length.) It has some crude but functional diff resolution, so that you can sync across multiple machines. Browsers _could_ provide better tooling to make this kind of app architecture more convenient to write; and theoretically you could imagine cloud providers providing some kind of standard for datastores. But there's no market for it; non techies really don't care about the location of their data to any great extent.