[css-pseudo] [css-variables] highlight pseudos and custom properties (closes #6264)

[delan]

This patch makes edits for #6264:

• adds a section stating that custom properties are allowed on all pseudos
• explicitly clarifies that non-inherited registered customs are also inherited

One thing I’m unsure of… should we restate or refer to this near the lists of applicable properties? Especially with the one for ::marker, which is in a separate spec, the reader could miss this if they’ve only read the section about a specific pseudo.

[fantasai]

The second half looks good.

For the first half, I think it would be better if we put this into the definition of custom properties. E.g.

- Applies to: all elements
+ Applies to: all elements and all pseudo-elements (including those with restricted property lists)

I think that'll be a less awkward place to put it, and makes it an integral part of the definition of exactly the thing it applies to.

[delan]

Thanks @fantasai, done in e0adc21. Do we need to pull Tab in as reviewer for the css-variables part?

[tabatkins]

r+ on the Variables edit