[css-content] attr() values are not defined

[heycam]

Examples in https://drafts.csswg.org/css-content/ mention attr(), but there is no definition in the spec for it, and the content property definition's value doesn't seem to allow it.

[xfq]

It looks like it is defined in V&U: https://drafts.csswg.org/css-values-4/#attr-notation