Those HTML attributes you never use
The graveyard of HTML attributes which never gained public attention/usage is large and full of useful ideas.My favourite was relational links such as     
     
       
     
      

Opera was the only browser which had a toolbar with these relational links. This was around mid-2000. Unfortunately, this toolbar never survived or got taken over by other browsers.Some relations survived (such as rel="search" for custom search pages), some got killed-by-ecosystem (such as RSS).