That looks alright. I probably prefer some evolution of B. At some
point CSS really needs to start thinking in terms of an object model
and a syntax. And the syntax just feeds the object model, upon which
everything else operates. So you wouldn't refer to 'url()' when
serializing, but rather a CSS URL value object. And that object has
some kind of internal slot (the local url flag), which we can also
expose API-wise perhaps, etc.