For background see http://enndeakin.wordpress.com/2009/10/05/visible-focus-rings/ https://bugzilla.mozilla.org/show_bug.cgi?id=418521 To sum it up we need a way to differentiate how an element was focused. The focusring pseudo class should be the platform specific pseudo class that we use to show the focus outline.
This has been added to the CSS Selectors draft as the :focusring pseudoclass https://github.com/w3c/csswg-drafts/pull/709
The name of the pseudoclass has changed in the meantime to :focus-visible. Sources: https://drafts.csswg.org/selectors-4/#the-focus-visible-pseudo https://caniuse.com/#feat=css-focus-visible https://bugzilla.mozilla.org/show_bug.cgi?id=1445482