title: CSS Margin-Top with Percentage -- Cross Browser Issue
body: <p>I'm working on an overlay in CSS and ran into a cross browser issue. The overlay height should be 80% of the viewport and be centered vertically. (80% height on the element, -40% top margin, 50% from top positioned absolutely). </p>
  
  <p>The fiddle below works in Chrome but not in Firefox.. the issue seems to be the percentage for margin-top. Make sure to resize your browser to see the full effect.</p>
  
  <p><strong>Fiddle:</strong> <a href="http://jsfiddle.net/DEn6r/1/" rel="nofollow">http://jsfiddle.net/DEn6r/1/</a></p>
  
  <p>Thank you for helping!</p>