I have no idea where to start doing this..
Have you checked that the new Heartbeat API in WordPress does not already perform this kind of autosave for user profiles (probably not but worth a lookyloo)?
Suppose you might be able to do something using this filter:
Like register a custom JS script which upon visiting the profile page creates a session cookie (or perhaps localstorage for html5). Then you run a timed function every 10 seconds or something which looks into all the fields and saves them by field name -> value..
Then when a person loads the edit page of their profile you check if the cookie is set and if so run through all values and input them in the approriate field (by field name).
This is just a quick idea on how to do it and I have no idea if it’s actually doable 🙂
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