title: How to add transitions/effects to display:block
body: <p>I've a div like this:</p>
  
  <pre><code>.x{
     ...
  }
  </code></pre>
  
  <p>And a sort of "submenu" initially hidden:</p>
  
  <pre><code>.x_submenu {
     ...
     display:none;
     ...
  }
  </code></pre>
  
  <p>The submenu will be visible only when the user is on the x div:</p>
  
  <pre><code>div.x:hover .x_submenu {display:block; }
  </code></pre>
  
  <p>Now, I'd like to make it visible with a transaction or an effect that makes the visibility more "slow".
  Is there a way to achieve that goal, possibly with a cross-browser solution?
  Thanks,
  A</p>