Some browsers does not asks the client if he want use the application cache, but simply just downloads the whole thing at first visist (e.g. the browser on android). That can cause troubles when the application cache is many MB's and the client is on a mobile network - that is expensive! And is it possible to stop the cache from being downloaded? Pressing the menu button on e.g. android will not close the browser, it will be running in the background.
Is it a good idea to only add the manifest based on a cookie set when the client pushes a "hey i want to offline cache this site" button? Will that cause any new challenges?
I've tested this, and it seems to work. Needs more testing though. Can provide a link to the site in about two weeks if anyone is interrested.