simple div shows in popup using javascript

<div id="popup" class="popup">
  This a vertically and horizontally centered popup.
</div>

<a onclick="showPopup('popup');">Show Popup</a>

<style type="text/css">
  .popup {
    width:200px;
    height:100px;
    position:absolute;
    top:50%;
    left:50%;
    margin:-50px 0 0 -100px; /* [-(height/2)px 0 0 -(width/2)px] */
    display:none;
  }
</style>

<script type="text/javascript">
  function showPopup(id) {
    var popup = document.getElementById(id);
    popup.style.display = 'block';
  }
</script>