Many of the supported border styles involve repeating artifacts: a dot, dash, or a combination of those. The user is allowed to control the thickness of the border feature numerically. But when it comes to the frequency - i.e. what's the distance between repetitions of the feature - the control is limited to a very few discrete options (and even with those, the relative frequencies are not clearly specified). We should be able to say exactly what distance - in absolute or feature-size-relative terms - should exist between repetitions of the feature.