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 1.5 on 2005-11-29 Note: Before Firefox 57, Firefox had a bug where ::before pseudo-elements were still generated, even if the content property value were set to normal or none. Before Firefox 3.5, only the CSS level 2 behavior of :before was supported, which disallowed position, float, list-style-* and some display properties.
In Firefox for Android version 4 on 2011-03-29 Note: Before Firefox 57, Firefox had a bug where ::before pseudo-elements were still generated, even if the content property value were set to normal or none.
In Internet Explorer version 9 on 2011-03-14
In Quest Browser version 5.0 on undefined
In Opera version 7 on 2003-01-28
In Opera Android version 10.1 on 2010-11-09
In Safari version 4 on 2009-06-08
In Safari on iOS version 3 on 2009-06-17
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 3 on 2009-06-17