Web Fonts are not actually cached in the offline.appcache file... it's something I've been meaning to fix, but it's a little complicated. If you manually put the URL to the webfont in the offline.appcache file it might work. It's a known problem though.
@Blacksmith - there are actually different storages involved in the browser, it's not all just one big cache. It goes something like this:
Ordinary document cache: stores HTML of recently visited pages, and cleared when the user hits 'clear cache'.
App Cache: stores files for Construct 2 games so the next time you play it loads instantly. *Not* cleared by pressing 'clear cache' - in fact there is no obvious way for the user to remove this at all.
WebStorage: stores data from the WebStorage object. *Not* cleared by pressing 'clear cache', like App Cache it's difficult for the user to manipulate. It's also commonly limited to 5-10mb.
The main problem is usually just the fact it's locally stored - so if you switch browser or computer, you can't get your data. The best way to solve that is store it server-side and use the AJAX object to communicate with the server.