This also implements the `:high-value` and `:low-value` that are in the
spec.
Same note as before about this being based on the very-drafty CSS Forms
spec. In fact, some of this isn't even in that spec yet. Specifically,
the `:suboptimal-value` and `:even-less-good-value` names are undecided
and subject to change. However, it's clear that this is a pseudo-class
situation, not a pseudo-element one, so I think this is still an
improvement, as it allows styling of the `::fill` pseudo-element
regardless of what state it is in.
Relevant spec issue: https://github.com/openui/open-ui/issues/1130
