Hello TAG!

I'm requesting a TAG review of the advanced version of the CSS attr() function.

The attr() CSS function is used to retrieve the value of an attribute of the selected element and use it in the stylesheet. There are two versions:

1. Basic version: Specified in CSS2. Allows usage in pseudo-element content property only as a string. This is already supported by all major browsers.
2. Advanced version: Specified in CSS Values and Units L4. Allows usage in all CSS properties, and can represent various types of values

• Explainer: https://developer.mozilla.org/en-US/docs/Web/CSS/attr#Examples
• Specification URL: https://drafts.csswg.org/css-values-4/#attr-notation
• Tests:
  • Existing WPT: https://wpt.fyi/results/css/css-values?label=master&label=experimental&aligned=&q=attr
  • Plan to add more: https://bugs.chromium.org/p/chromium/issues/detail?id=1081995
• Primary contacts:
  • CSSWG for the specification
  • Xiaocheng Hu (xiaochengh), Google for the Chromium implementation
• Organization driving the specification: CSSWG
• External status/issue trackers for this specification:
  • Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=246571
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=26609
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=435426

You should also know that...

In CSS4, the feature has been substantially revised to ease implementations. Most importantly, it now works similar to var(), namely, attr() is considered valid at parse time, and the actual validation of the attribute value is deferred to computed value time. Please review this version.

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