the current outline: thin dotted that is forced as "default" is not in fact default in Firefox/OS X; forcing it leads to unsightly dotted outline that's non-standard for the browser/platform.
removing it seems to have no adverse effect in other all situations where this mixin is used (buttons, scaffolding for general link styles, and the case of forms). attempted to use outline: initial at first, but this seems somehow broken in Firefox (ends up removing standard Firefox dotted outline on links).
This is mostly a bug-fix, rather than a new feature (hence targetting it for v3)