If you read closer: CSS Values and Units Level 3 adds the ability to use attr() on any CSS property, not just content, and to use it for non-string values (e.g

If you read closer: CSS Values and Units Level 3 adds the ability to use attr() on any CSS property, not just content, and to use it for non-string values (e.g. numbers, colors). It's not attr that's unusable, but rather being able to use it on any arbitrary CSS property. Though I'm surprised they don't have a separate entry for just the attr function alone.