On the standards track: Yes
Is Experimental: No
Is Deprecated: No
In Chrome version 1 on 2008-12-11
In Chrome Android version 18 on 2012-06-27
In Edge version 12 on 2015-07-29 Note: Before Edge 16, Microsoft Edge treats all unknown elements (such as custom elements) as the same element type.
In Firefox version 3.5 on 2009-06-30
In Firefox for Android version 4 on 2011-03-29
In Internet Explorer version 9 on 2011-03-14 Note: Internet Explorer treats all unknown elements (such as custom elements) as the same element type.
In Quest Browser version 5.0 on undefined
In Opera version 9.5 on 2008-06-12
In Opera Android version 10.1 on 2010-11-09
In Safari version 3.1 on 2008-03-18
In Safari on iOS version 2 on 2008-07-11
In Samsung Internet version 1.0 on 2013-04-27
In WebView Android version 2 on 2009-10-26
In WebView on iOS version 2 on 2008-07-11