XML served by ws/2 does not conform to the RelaxNG schema: https://github.com/metabrainz/mmd-schema/blob/master/schema/musicbrainz_mmd-2.0.rng

Two, related violations:
https://github.com/metabrainz/mmd-schema/blob/c45e3fa26c5633db48f65cace9491ffcfbd62ee4/schema/musicbrainz_mmd-2.0.rng#L454
This requires `primary-type` elements to have an `id` attribute.

https://github.com/metabrainz/mmd-schema/blob/master/schema/musicbrainz_mmd-2.0.rng#L465
This requires `secondary-type` elements to have an `id` attribute.

Example of non-conformant XML:
https://musicbrainz.org/ws/2/release/?query=artist%3A%22A%20Winged%20Victory%20for%20the%20Sullen%22%20AND%20release%3A%22Iris%22