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

[andruud]

As briefly discussed with @tabatkins during CSS Day:

• In @property, we have "<length>", and similar.
• We intend to use the same for the parameter types of custom functions (probably).
• We have a loose plan of dropping the string-part of the syntax string, making it possible to specify <length> directly. (https://github.com/andruud/csswg-drafts/pull/2/files)

Why doesn't attr() use the same way of specifying types? I.e., shouldn't we do attr(<length> em, 0px) instead of attr(length em, 0px)?

EDIT: I accidentally posted the issue template.