// JavaScript Document
// Browser safe opacity handling function

function setOpacity( value ) {
 document.getElementById("styled_popup").style.opacity = value / 10;
 document.getElementById("styled_popup").style.filter = 'alpha(opacity=' + value * 10 + ')';
}

function fadeInMyPopup() {
 for( var i = 0 ; i <= 100 ; i++ )
   setTimeout( 'setOpacity(' + (i / 10) + ')' , 8 * i );
}

function fadeOutMyPopup() {
 for( var i = 0 ; i <= 100 ; i++ ) {
   setTimeout( 'setOpacity(' + (10 - i / 10) + ')' , 8 * i );
 }

 setTimeout('closeMyPopup()', 800 );
}

function closeMyPopup() {
 document.getElementById("styled_popup").style.display = "none"
}

function fireMyPopup() {
 setOpacity( 0 );
<!-- Due to different browser naming of certain key global variables, we need to do three different tests to determine their values -->

// Determine how much the visitor had scrolled

var scrolledX, scrolledY;
if( self.pageYOffset ) {
  scrolledX = self.pageXOffset;
  scrolledY = self.pageYOffset;
} else if( document.documentElement && document.documentElement.scrollTop ) {
  scrolledX = document.documentElement.scrollLeft;
  scrolledY = document.documentElement.scrollTop;
} else if( document.body ) {
  scrolledX = document.body.scrollLeft;
  scrolledY = document.body.scrollTop;
}

// Determine the coordinates of the center of browser's window

var centerX, centerY;
if( self.innerHeight ) {
  centerX = self.innerWidth;
  centerY = self.innerHeight;
} else if( document.documentElement && document.documentElement.clientHeight ) {
  centerX = document.documentElement.clientWidth;
  centerY = document.documentElement.clientHeight;
} else if( document.body ) {
  centerX = document.body.clientWidth;
  centerY = document.body.clientHeight;
}

  var leftOffset = scrolledX + (centerX - 900) / 2;
  var topOffset = scrolledY + (centerY - 400) / 2;

  document.getElementById("styled_popup").style.top = topOffset + "px";
  document.getElementById("styled_popup").style.left = leftOffset + "px";
 
 document.getElementById("styled_popup").style.display = "block";
 fadeInMyPopup();
}