[css-values] attr()'s type system is inconsistent with other things

[mozilla-apprentice]

A resolution was made for csswg-drafts/#10437.

[css-values] attr()'s type system is inconsistent with other things

• RESOLVED: Use the <syntax> production that we hae defined in the mixins spec, consistent with what we will be allowing in registered custom properties

Discussion.

---

To file a bug automatically for these resolutions, add the bug label to the issue.

If no bug is needed, the issue can be closed.