> What about an alist?
>
> '((5 some-face)
>   (10 some-other-face)
>   (100 yet-another-face))

There are many things that could be useful, but Eli already said several months ago that he doesn't want to add bells and whistles that would make redisplay slower and more fragile.

My patch implements (more or less) something that he said back then that it would be acceptable.