> Correct; the user or user-agent can always delete data, just like in desktop or mobile apps. The only truly reliable storage is on your servers.

That makes an entire concept of true offline applications very dubious.
Maybe we should reverse the idea: let UA ask apps how much space they could free without losing any valuable content (probably with a textual message regarding what exactly app considers to be a valuable content). Then UA may present to user a statistics, how much space each application uses for caches and for documents, and user can decide what data he wants to delete, just like it happens with regular applications.

> Indeed!

Why not include this functionality in Quota API?