Add attr() fallback values subfeature

[Elchi3]

Summary

The attr() function supports fallback values. Add a subfeature for that.

Test results and supporting details

• https://developer.apple.com/documentation/safari-release-notes/safari-18_4-release-notes
• https://bugzilla.mozilla.org/show_bug.cgi?id=1448248
• https://chromestatus.com/feature/4680129030651904

Related issues

• I should file a content issue so that https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Values/attr gets updated. (experimental banner should be removed).
• Chrome 133 supports more attr functions #28661 did more Chrome 133 attr() updates.

[github-actions]

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