new `attr()` function with `type(<length>)` falsely reports syntax error

[ericrav]

Using the redesigned css attr() function displays a syntax error:

#foo {
  left: attr(x type(<length>));
}

Results in:

• at first <: ) expected css(css-rparentexpected)
• at closing }: at-rule or selector expected css(css-ruleorselectorexpected)

Details about the spec:

• https://drafts.csswg.org/css-values-5/#attr-notation
• https://developer.chrome.com/blog/advanced-attr
• https://developer.mozilla.org/en-US/docs/Web/CSS/attr

This is pretty new and experimental feature in CSS, so I understand why it's not supported. Happy to try to implement if help is welcome.

[8Crafter]

I really hope this gets fixed soon as it is really annoying.

[chloeburroughs]

I would also really like to see this added very soon, also with support for the applicable @supports query:
@supports (x: attr(x type(*))) {}, which is the one advised by CSS-Tricks here: attr() | CSS-Tricks.

Doing so would get me closer to being able to remove unsafe-inline from my CSP with regards to stylesheets.