Properties color and background-color should be motion-capable with CSS Transitions. Aside from being neat eye-candy, this would particularly enhance the branding opportunities that are the core utility of ::selection.
This idea should probably be discussed on email@example.com (the W3C public mailling list for the CSS Working Group) - could you please describe the enhancement in an email to the list?
Yeah, it was discussed when this ticket was originally created. As typical of standards group mailing lists, it was smothered into non-existence by the denials of people with more important email domains than mine.