The law appears to ban only long term cookies, so you are still ok if you use session cookies (such as those used for logins) which expire when the browser is closed.

Now comes the hacks: serve javascript with a unique id in it. Set the cache policy to never re-download, or recheck the file. Use you lawful (semi) permanent cookie.

The user obviously asked for the data to be stored, since it's part of caching.

Or am I going to have to ask the user before being allowed to cache anything locally?