How can I check if a page is available offline in HTML5?

I want to create a magazine reader in HTML5. One feature I'd like to add is ability to read articles while being offline.

I'd like to show a li开发者_如何学Pythonst of previously accessed articles and still available offline. The problem is that I don't know if it's possible to check if a given URL is available in a user's offline cache or not. I could store previously accessed URLs in local storage but I can't be sure if these URLs are still available in the cache. Is it possible to be notified when a given URL is removed from the cache or check if the URL is available offline?

Thanks, Michal

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There's no way in the HTML5 spec. Firefox has an experimental mozItems property of window.applicationCache, but it seems to not work: always has a length of 0. I don't know if webkit has anything similar.