function getViewportSize()
	{
	 var size = [0, 0];
	
	 if (typeof window.innerWidth != 'undefined')
	 {
	   size = [
		   window.innerWidth,
		   window.innerHeight
	   ];
	 }
	 else if (typeof document.documentElement != 'undefined'
		 && typeof document.documentElement.clientWidth !=
		 'undefined' && document.documentElement.clientWidth != 0)
	 {
	   size = [
		   document.documentElement.clientWidth,
		   document.documentElement.clientHeight
	   ];
	 }
	 else
	 {
	   size = [
		   document.getElementsByTagName('body')[0].clientWidth,
		   document.getElementsByTagName('body')[0].clientHeight
	   ];
	 }
	
	 return size;
	}


function centerVertically(){
	var intro = document.getElementById('introduction');
	var viewport = getViewportSize();
	
	var marginTop = (viewport[1] - 380) / 4;
	intro.style.marginTop = marginTop + "px";

}

window.onresize = centerVertically;
window.onload = centerVertically;