Anne van Kesteren suggests:
> I would love it if CSS just exposed some host-defined slots (or required
> those to be defined on "document" or wherever) to make this more
> explicit. Would attr() ever be different from attribute selector
> matching for instance?
For now I suppose we can add attr() in HTML (with the same rules as
attribute selectors) but we can investigate better ways of handling this,
in particular if CSS will add new features that should have these rules...