A21: restrictions on attribute values?

On 9 October 1996, the ERB will vote to decide the following question.
A non-binding preliminary vote indicates the ERB is now leaning to Yes.

A.21 like SGML, XML will forbid empty strings as attribute values for
non-CDATA attributes, require FIXED attributes to take their default
values (,, and distinguish implied values from
null-string values (11.3.4).