Intent to prototype and ship: CSS attr() fallback

[CanadaHonk]

As of today, I intend to turn on the CSS attr() function fallback by default on all platforms. It has not been developed behind a flag, rather being enabled by default in the implementation patch as it is a low-risk feature. No other browsers ship or implement the feature at this time.

Summary:
The CSS attr() function allows the values of attributes to be used in the content property. CSS Values 5 adds a fallback with the syntax "attr(name, fallback)" which uses the fallback as a default value when the attribute does not exist in the target element. Please note that this does not enable the new var-like usage in all properties (only in content still) or types, this only adds the fallback value feature.

Bug: https://bugzil.la/1448248
Specification: https://drafts.csswg.org/css-values-5/#attr-notation
Standards body: W3C CSSWG
Platform coverage: All

Preference: None
DevTools bug: https://bugzil.la/1854104
Standards position: https://github.com/mozilla/standards-positions/issues/885

Other browsers:
- Blink: Not shipped or implemented. Bug: https://crbug.com/246571 (general)
- WebKit: Not shipped or implemented. Bug: https://bugs.webkit.org/show_bug.cgi?id=26609 (general)

Web platform tests: Added in Gecko implementation patch (https://phabricator.services.mozilla.com/D176801)