Add feature for CSS Typed OM

[queengooborg]

This PR adds a feature for the CSS Typed OM Level 1.

[jamesnw]

I'd propose a different approach- many of these keys should be part of separate features. For instance, api.CSS.cap_static should be part of the cap feature, similar to how the highlight feature contains api.CSS.highlights_static.

This feature should contain the keys that are related to CSS syntax and wouldn't be part of other features. Some examples-

• api.CSSNumericArray
• api.CSSNumericValue
• api.CSSUnitValue.value

Some of the keys here can be added to existing features, but there are others like rcap that will still need to be created.

[jamesnw]

I'm taking another look at this, and would appreciate input from others.

The Typed OM is not supported by Firefox yet, so if the Typed OM keys go with their corresponding properties, we will need to compute the status on each to ignore the typed OM keys.

Alternatively, we can have a Typed OM feature as proposed here, that tells a clearer support story, but separates api.CSS.cap_static from css.types.length.cap.

Any thoughts on where these keys should go?

[ddbeck]

This feature should contain the keys that are related to CSS syntax and wouldn't be part of other features.

This is the core thing. I think a lot of the keys here could survive as a typed OM feature no matter what.

The feature-specific keys, those are the troublesome ones. I'm going to make a pragmatic suggestion: put the per-feature keys into a draft feature—call it the typed OM "junk drawer" if we must. Sort the keys by which "real" feature they ought to live with and comment the lines accordingly.

I think this is going to be one of the prime features for resolving #971. We'll need some way to declare inferior or interior or adjacent features and this seems like a likely candidate for taking it on, since we'll get a bunch of them at once. If we can deal with that soon, then great. If we get impatient, then we'll have done the work to decide where they'll need to be moved already.

[jamesnw]

Another option would be to pull out a CSS numeric factory functions feature, which would encompass all of items that would be part of another feature. This would essentially be the draft junk drawer idea, and we could add comments saying that, but it would also be a useful feature. See #2000 for a draft PR.

[ddbeck]

Some adjustments needed on the description, but otherwise I think this is good to go now.

[jamesnw]

@queengooborg I shepherded this over the finish line since I had made some parallel changes in the interim- thanks for the initial feature and patience!