This is pretty low down on the priority list for primitives, but it's worth noting that the list of which elements are considered "links" is fixed forever as a[href], area[href], link[href]. We're getting around that partially by applying our custom "UA styles" to custom-a[href], but maybe it'd be worth talking about letting these UA-defined pseudo-classes be redefined, especially once @tabatkins's custom pseudos spec takes off.