There are two separate user desires that you need to separate: 1. If [browser/server/electric utility/network] crashes I should not lose work. 2. I want to ex

There are two separate user desires that you need to separate: 1. If [browser/server/electric utility/network] crashes I should not lose work. 2. I want to explore a few quick changes and then revert back to a known good point. The technical solution is to do continuous saving but let the user mark certain spots in the revision tree as more important. Please note that you will be storing all revisions and the structure will most definitely be a tree (due to undo/redo/forking whatnot) Marking could be as easy as hitting save or creating a named revision. From an UX standpoint continuous saves are best effort while explicitly hitting save should freeze the system and only return an OK once the data has really been persisted to disk. My $.2, I've worked on Google Docs and Tinkercad.. :-)