The xliff-spec designates these states as allowed, together with user-defined attributes prefixed with 'x-'. For the moment I only allow the predefined states, not the user-defined ones.
More info: http://docs.oasis-open.org/xliff/v1.2/os/xliff-core.html#state

Do you want me to also allow the user-defined attributes or just allow for arbitrary values (cfr. my reply above)?