Re: [ServiceWorker] window.fetch (#581)

`<iframe>` can be abused in Firefox, which doesn't fire a load event for hosts that don't resolve. Chrome still fires a load event. Test: https://jsbin.com/nuvaso/edit

`<img>` can be used fairly reliably due to `/favicon.ico` https://jsbin.com/sumuji/1/edit?html,js,output

Appcache can be used, by setting the external URL as an explicit cache entry and seeing if the cache succeeds.

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https://github.com/slightlyoff/ServiceWorker/issues/581#issuecomment-70121238