Drupal Lightbox2 fails W3C validation because of rel="lightbox[field_image][]" attribute
You can copy the lightbox2-insert-image.tpl.php file into your own theme and change it there - but this will break the functionality of the module as javascript uses rel attribute multiple times. 

You could refactor the javascript, change all occurrences where it accesses 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&amp;charset=%28detect+automatically%29&amp;doctype=Inline&amp;group=0)