title: position absolute & margin bottom
body: <p>I have this container which is absolutely positioned .. The problem with it is I can't give it a margin bottom value !! </p>
  
  <p>I want no matter how much content inside that green container .. I want the next div (under) to be under it with 30px always</p>
  
  <p>I can't do that and I don't know why !</p>
  
  <p><a href="http://jsfiddle.net/nxFkG/54/" rel="nofollow">http://jsfiddle.net/nxFkG/54/</a></p>
  
  <p>html</p>
  
  <pre><code>&lt;div class="bg-black"&gt;
      &lt;div class="bg-green"&gt;
          &lt;ul&gt;
              &lt;li&gt;lorem ipsum&lt;/li&gt;
              &lt;li&gt;lorem ipsum&lt;/li&gt;
              &lt;li&gt;lorem ipsum&lt;/li&gt;
              &lt;li&gt;lorem ipsum&lt;/li&gt;
              &lt;li&gt;lorem ipsum&lt;/li&gt;
              &lt;li&gt;lorem ipsum&lt;/li&gt;
              &lt;li&gt;lorem ipsum&lt;/li&gt;
              &lt;li&gt;lorem ipsum&lt;/li&gt;
              &lt;li&gt;lorem ipsum&lt;/li&gt;
              &lt;li&gt;lorem ipsum&lt;/li&gt;
          &lt;/ul&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  
  &lt;div class="under"&gt;
      &lt;h1&gt;THIS SHOULD BE UNDER THE GREEN CONTAINER NO MATTER HOW MUCH THE CONTENT INSIDE IT&lt;/h1&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>css</p>
  
  <pre><code>.bg-black {
      background-color: #000;
      position: relative;
      width: 160px;
      height: 120px;
  }
  .bg-green {
      width: 120px;
      position: absolute;
      background-color: green;
      margin-bottom:163px;
  }
  </code></pre>
  
  <p>Any help on how to do it &amp; why it happend !! </p>
  
  <p>thank you</p>