function PopupCentrata(apri) {
   var w = 575;
   var h = 455;
   var x = Math.floor((screen.width-w)/10);
   var t = Math.floor((screen.height-h)/10);
	   var stile = "top=" + t + ", left="+ x + ", width=" + w + ", height=" + h + ",toolbar=yes, scrollbars=yes, location=yes, statusbar=yes, menubar=yes, resizable=yes";
      window.open(apri,"",stile);
 }
<!-- <a href="javascript:PopupCentrata('cartomanzia-testimonianze.html')">Apri la popup</a> -->
<!-- <form>
<!-- <input type=button value="Open the Popup Window" onClick="javascript:popUp('merda')">
<!-- </form>