Intent to Ship: Advanced CSS attr() function.

[Diego Escalante]

As of Firefox 155, I intend to turn advanced attr() on by default on all platforms. It has been developed behind the layout.css.attr.enabled preference.
Status in other browsers:

• Blink - Shipped.
• WebKit - Enabled as preview 

Bug to turn on by default: https://bugzilla.mozilla.org/show_bug.cgi?id=2038940
Link to standards-positions discussion: https://github.com/mozilla/standards-positions/issues/1143
Intent to Prototype Thread: https://groups.google.com/a/mozilla.org/g/dev-platform/c/9tQ-XV82EqU/m/_-NCVEYzAgAJ
TAG Review: https://github.com/w3ctag/design-reviews/issues/513