title: CSS animation : move a div with the new position relative to the previous
body: <p>for better performance, I want replace:</p>
  
  <pre><code>$('#foo').animate({ left: '+=42px' }, 500);
  </code></pre>
  
  <p>by a transition (or animation) CSS3.
  But how can we do to implement "+=" on left property in CSS? </p>
  
  <p>How move a div with the new left position relative to the previous?</p>
  
  <p>thx.</p>