Currently align-self and justify-self are defined to allow [ <overflow-position>? && <content-position> ]. That these keywords allow reordering makes parsing ambiguous in the shorthand, so they are disallowed from the shorthand. However, we could fix the order of the keywords, and that would make them unambiguous in the shorthand. It would, however, violate the "allow reordering unless unambiguous" principle for the longhands.