You can copy the
rel attribute multiple times.
rel and change it to
data-rel and create patch and submit it to drupal.org to the author of the issue as a request (or just submit issue now requesting this and wait if/until the maintainer does it).
On a personal note - why do you need it to pass W3C validation? Is it a project MUST requirement? If not, I would not waste any time on this as valid HTML is not that important any more, not even for SEO and indexing.
Edit: http://google.com is not passing W3C validation, I think that sends a message. (http://validator.w3.org/check?uri=google.com&charset=%28detect+automatically%29&doctype=Inline&group=0)