On the standards track: Yes
Is Experimental: No
Is Deprecated: No
In Chrome version 4 on 2010-01-25
In Chrome Android version 18 on 2012-06-27
In Edge version 12 on 2015-07-29
In Firefox version 3.5 on 2009-06-30 Note: Since Firefox 4 the browser returns true when 'Work Offline' mode is disabled and false when it is enabled, regardless of actual connectivity. Since Firefox 41, on OS X and Windows, the returned values follow the actual network connectivity, unless 'Work offline' mode is selected (where it will always return false).
In Firefox for Android version 4 on 2011-03-29
In Internet Explorer version 10 on 2012-10-26
In Quest Browser version 5.0 on undefined
In Opera version 10.6 on 2010-07-01 Note: From Opera 11.1 until Opera 12.1, the browser returns true when 'Work Offline' mode is disabled and false when it is enabled, regardless of actual connectivity.
In Opera Android version 11 on 2011-03-22 Note: From Opera 11.1 until Opera 12.1, the browser returns true when 'Work Offline' mode is disabled and false when it is enabled, regardless of actual connectivity.
In Safari version 4 on 2009-06-08
In Safari on iOS version 5 on 2011-10-12
In Samsung Internet version 1.0 on 2013-04-27
In WebView Android version 4.4 on 2013-12-09 Note: Can incorrectly return true, see bug 41369933.
In WebView on iOS version 5 on 2011-10-12