1.3, Notation: SGML? Reference Concrete?

1.3.a Should we assume that saying links are expressed using SGML elements
and attributes, and describing them in SGML terms, is a satisfactorily
complete syntax spec, thus avoiding a requirement for any BNF?

1.3.b If links are SGML elements, should require XML-style reference
concrete syntax + quoted attribute values?  I.e. assert that
all links would fit into XML docs?