AFAIK in Firefox and Internet Explorer, this property doesn't check the present connection state, it only check if browser is in the offline mode. After switching the browser to offline mode (FIle -> Work offline) it has a false value, all other conditions return a true value; 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.