HTML5 provides APIs to let user check if user_agent is online or offline, but unfortunately different browser vendors implement differently. From my testing, Chrome is the only browser which detects network connectivity and provides "correct" online/offline information. While Firefox etc is checking if user_agent is in "Work Offline" mode, so it returns true even though network connectivity is lost. Due to this inconsistency across browsers, the two "ideal" ways (check navigator.onLine, or window.ononline/onoffline event) will NOT work.
There are two possible solutions to overcome this and provide online/offline status.
1. AppCache error event (applicationCache.onerror, applicationCache.addEventListener('error')) - pay attention to reaching applicationCache size limit.
2. XHR (long polling predicable resource on server, if timeout either server is down or network connectivity is lost)
reference to http://www.html5rocks.com/en/mobile/workingoffthegrid
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