Bottom line, will navigator.onLine tell me if the user is unable to contact there server due to poor coverage even if 3G/4G and WiFi are turned on and they are not in airplane mode or anything? Just your average dead zone.
From the MDN article about
Browsers implement this property differently.
In Chrome and Safari, if the browser is not able to connect to a local area network (LAN) or a router, it is offline; all other conditions return true. So while you can assume that the browser is offline when it returns a false value, you cannot assume that a true value necessarily means that the browser can access the internet. You could be getting false positives, such as in cases where the computer is running a virtualization software that has virtual ethernet adapters that are always "connected." Therefore, if you really want to determine the online status of the browser, you should develop additional means for checking. To learn more, see the HTML5 Rocks article, Working Off the Grid.
In Firefox and Internet Explorer, switching the browser to offline mode sends a false value. All other conditions return a true value
According to MDN, it varies on the browser for implementation.
I would assume with WebKit browsers, its not a reliable way to know that a browser has Internet access, just some form of network access.
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