In CSS Forms we have a shape for the elements of a meter:

<meter>
├─ ::slider-track
 │  └─ ::slider-fill

However, for this to be useful for styling we need some way to style based on how optimal the value of the meter element is.

WebKit (and blink) uses different pseudo elements instead of a single :slider-fill equivalent and their naming is ::-webkit-meter-even-less-good-value, ::-webkit-meter-suboptimum-value and ::-webkit-meter-optimum-value.

Firefox has a single ::-moz-meter-bar pseudo element and instead applies pseudo classes to <meter> these are :-moz-meter-sub-sub-optimum, :-moz-meter-sub-optimum and :-moz-meter-optimum.

Using pseudo classes seems the better approach between the two. I suspect we can drop the meter prefix and just go with something like :sub-sub-optimum, :sub-optimum, :optimum? But naming would need bikeshedding as neither Firefox's nor WebKit's seem great.