I think that the HREFLANG attribute[1] should be allowed to be empty as well, just like xml:lang is allowed to be empty.
[1]<http://atompub.org/2005/04/18/draft-ietf-atompub-format-08.html#rfc.section.4.2.9.4>
-- Anne van Kesteren <http://annevankesteren.nl/>
