Maybe we now need some examples to balance the SBS one where there is
no requirement or applicability of an emphasis on instance validity.
I already gave you one: XSLT. The spec does not impose requirements on
the stylesheets or any other documents, only on the behavior of a
piece of software, the XSLT processor. Validity of a stylesheet is
specified indirectly by specifying what a processor must do when
presented with a document that purports to be a stylesheet but does not
follow the rules.