JavaScript - Funcion JavaScript para Posicionar pagina en Sitio especifico

<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>

<div id="popup" class="popup">

  This a vertically and horizontally centered popup.

</div>

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

<script type="text/javascript">

  function showPopup(id) {

    var popup = document.getElementById(id);

    popup.style.display = 'block';

  }

</script>