The url('  ...') is boilerplate that unfortunately has to be there as
the WG were having an off-day and copied CSS syntax into XSL FO

I actually found this useful, it simplifies the development of
transformations from (X)HTML to XSLFO and vice versa quite a bit.

If they only had made this a bit more explicit, rather than suggesting
in section 5 of the spec "there's only one grammar for generating valid
values for FO properties..."