title: margin-top not working with margin: 0 auto
body: <p>I have a relative positioned div which I want to horizontal center. Also I want it to be at certain height in the middle of page. But with css below, I could put it in middle horizontally but it does not apply margin-top to it</p>
  
  <pre><code>  position:relative;
    margin: 0 auto;
    text-align:center;
    color:white;
    opacity:0.75;
    margin-top:30px;
  </code></pre>
  
  <p>What could be wrong??</p>