feat: allow relative-size and global values in relative-font-units

[xbinaryx]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

To update the relative-font-units rule to more clearly and strictly enforce the use of relative font units, while always allowing relative-size keywords and global values, and always disallowing absolute-size keywords.

What changes did you make? (Give an overview)

• Updated the rule logic to:
  • Always allow the smaller and larger relative-size keywords.
  • Always allow global values such as inherit, initial, revert, revert-layer, and unset.
  • Always disallow absolute-size keywords (xx-small, x-small, small, medium, large, x-large, xx-large, xxx-large) and the math keyword.
• Updated tests and documentation to reflect the new rule behavior.

Related Issues

Fixes #193

Is there anything you'd like reviewers to focus on?

[Tanujkanti4441]

Thanks for the PR! Everything looks good, just a minor change request.

[nzakas]

Thanks for taking a look at this.

There is no such thing as a math keyword in CSS, there are only math functions. Please update the logic and docs to take this into account.

[xbinaryx]

@nzakas font-size does accept a math keyword, distinct from math functions. It's listed by MDN as part of the font-size syntax.

[nzakas]

Huh. I didn't realize that was a literal value, I thought it was a reference to the math functions. TIL. 👍

[nzakas]

LGTM. Thanks!