title: jQuery add css value incrementally
body: <p>I want to add or subtract the value of 'top' in this css change. Rather then specifying a specific ammount of pixels I would like to add 20px to the current value provided by the external css.</p>
  
  <p>The goal is to make the list move up an down.</p>
  
  <pre><code>$('.up').click(function(){
     $('.list').css('top','-20px');
  });
  </code></pre>
  
  <p>the HTML</p>
  
  <pre><code>&lt;div id="container"&gt;
      &lt;ul class="list"&gt;
          &lt;li &gt;first item&lt;/li&gt;
          &lt;li class="active"&gt;second item&lt;/li&gt;
          &lt;li&gt;third item&lt;/li&gt;
          &lt;li &gt;fourth item&lt;/li&gt;
      &lt;ul&gt;
  &lt;/div&gt;   
  &lt;a href="#" class="up"&gt;up&lt;/a&gt;
  &lt;a href="#" class="up"&gt;down&lt;/a&gt;​
  </code></pre>