Those HTML attributes you never use
The graveyard of HTML attributes which never gained public attention/usage is large and full of useful ideas.<p>My favourite was relational links such as<p><pre><code>     <link rel='first' href='/de' title='Homepage' />
     <link rel='prev' href='/some/bunnies.php' title='Previous Page about bunnies' />
     <link rel='next' href='/some/cats.php' title='Next page about cats' />  
     <link rel="copyright" href="/de/impressum.php" title="Impressum">
     <link rel="alternate" href="/gr/samepage.php" hreflang="gr" title="Page name in greek"> 
</code></pre>
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.<p>Some relations survived (such as rel="search" for custom search pages), some got killed-by-ecosystem (such as RSS).