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
In Firefox version 1 on 2004-11-09
In Firefox for Android version 4 on 2011-03-29
In Internet Explorer version 7 on 2006-10-18 Note: Before Internet Explorer 10, the combinator only works in standards mode. Internet Explorer 7 doesn't update the style correctly when an element is dynamically placed before an element that matched the selector. In Internet Explorer 8, if an element is inserted dynamically by clicking on a link the first-child style isn't applied until the link loses focus.
In Quest Browser version 5.0 on undefined
In Opera version 3.5 on 1998-11-18
In Opera Android version 10.1 on 2010-11-09
In Safari version 1 on 2003-06-23
In Safari on iOS version 1 on 2007-06-29
In Samsung Internet version 1.0 on 2013-04-27
In WebView Android version 4.4 on 2013-12-09
In WebView on iOS version 1 on 2007-06-29