title: validation error on prettyPhoto's rel attribute
body: <p>It is looks like W3C's validator return a validation error on prettyPhoto's rel attribute for HTML5 pages. How do I solve this error?</p>
  
  <p><em><strong>Bad value prettyPhoto[gallery1] for attribute rel on element a: Keyword prettyphoto[gallery1] is not registered.</em></strong></p>
  
  <p>Many thanks!</p>