Add heading selectors

[keithamus]

This adds the :heading and :heading() pseudo classes as specified in https://drafts.csswg.org/selectors-5/#headings, and added in BCD 6.1.1: mdn/browser-compat-data#27517

Refs #3218 which updated BCD to 6.1.1. Perhaps it didn't add these as they're not in a released browser yet?

[captainbrosset]

This looks great to me, modulo the suggestion for the name, and missing period at the end of the description.

[ddbeck]

Thanks for adding this! I have a question in a line comment about the coverage of this feature and how to describe it.

I also have one general question: do we know anything about how these might be implemented across engines? If it seems very likely that they might land separately (e.g., that a vendor might implement :heading but not :heading(), it might be desirable to create independent features for each pseudo.

Otherwise, as one feature seems fine with me.

[ddbeck]

I think it would be nice to be more specific here. Something like:

Suggested change

	description: The `:heading` and `:heading()` CSS pseudo-classes match headings of varying levels.
	description: The `:heading` and `:heading()` CSS pseudo-classes match headings. The `:heading` pseudo-class matches any `<h1>` through `<h6>`. The `:heading()` pseudo matches…

However, I don't really know how to fill in the rest. I read the spec and I read the MDN page (which seem to conflict greatly) and I don't get it. I suspect there's some history here, so giving a clue about what behavior we're talking about is pretty important.

[keithamus]

Does this new description work?

[keithamus]

do we know anything about how these might be implemented across engines?

It is implemented behind a flag in Firefox nightly right now. WebKit are supportive of this and its related feature (WebKit/standards-positions#523). So I am hopeful.

If it seems very likely that they might land separately

I am unsure if it'll land separately, it seems unlikely, however these two features represent the ones found in BCD.

I read the spec and I read the MDN page (which seem to conflict greatly) and I don't get it.

The functional form underwent some minor changes. It used to be specced that you could use An+B notation e.g. :heading(odd), :heading(3n+1) but it was altered to only accept a list of integers now, so :heading(1, 3, 5, 7, 9) and :heading(1, 4, 7) would be the equivalent selectors for those.

[ddbeck]

Some extremely minor suggestions on the new description, but I'm happy with this. Thank you, @keithamus!

[ddbeck]

Tiny nits on the description (to avoid future tense, to avoid non-literal code, and align the example with other descriptions), but like this a lot. Thank you!

Suggested change

	description: The `:heading` and `:heading()` CSS pseudo-classes match headings of varying levels. The `:heading` pseudo-class matches any heading (`<h1>` through `<h6>`). The `:heading(<integer>#)` selector matches any of the given levels, for example `:heading(1, 2)` will match headings level 1 and 2.
	description: The `:heading` and `:heading()` CSS pseudo-classes match headings of varying levels. The `:heading` pseudo-class matches any heading (`<h1>` through `<h6>`). The `:heading()` selector matches any of the given levels. For example, `:heading(1, 2)` matches headings level 1 and 2.