We've been thinking about the best way to eventually handle this.. save to a server, local storage or a cookie. Cookies are probably a bad idea though since they can expire and be deleted when the user clears settings.
I'd say local storage, but be able to chose the save path. Then I could back up my save to google drive. Actually if there was a way to save it directly to google drive or dropbox that'd be really cool.
At least my google cloud folder has to sync up with the local disk first so it's not really saving directly to it (not sure how dropbox works since I don't have it). I just wasn't sure if you could say log into your cloud storage and allow another website have access to it.