HTML describes the "centered alignment" of a dialog element in somewhat opaque prose.

Blink's implementation of this is a hack in their rendering code

This needs to be described in terms of CSS (ideally as some rules in the UA stylesheet). Apparently it's not implementable in CSS, which is a problem. First, UAs should not have to add custom layout code for the dialog element, and second authors should be able to achieve the same positioning behavior for their own elements.