//call by
//<a href="javascript:changeFontSize(5)">Increase Font Size</a> 
//<a href="javascript:changeFontSize(-5)">Decrease Font Size</a>

function changeFontSize(inc)
{
	
	for(var i=0; i<=20; i++){
	  if (i==0)			{	var thiselement = 'p';}
	  else if (i==1)  	{	var thiselement = 'body';}
	  else if (i==2)	{	var thiselement = 'div';}
	  else if (i==3)  	{	var thiselement = 'span';}
	  else if (i==4)  	{	var thiselement = 'li';}
	  else if (i==5)  	{	var thiselement = 'a';}
	  else if (i==6)  	{	var thiselement = 'td';}
	  else if (i==7)  	{	var thiselement = 'th';}
	  else if (i==8)  	{	var thiselement = 'blockquote';}
	  else if (i==9)	{	var thiselement = 'h1';}
	  else if (i==10)	{	var thiselement = 'h2';}
	  else if (i==11) 	{	var thiselement = 'h3';}
	  else if (i==12)  	{	var thiselement = 'h4';}
	  else if (i==13)  	{	var thiselement = 'h5';}
	  else if (i==14)  	{	var thiselement = 'h6';}
	  else if (i==15)  	{	var thiselement = 'h7';}
	  else if (i==16)  	{	var thiselement = 'h8';}
	  else if (i==17)  	{	var thiselement = 'h9';}
	  else if (i==18)  	{	var thiselement = 'h10';}
	  else if (i==19)  	{	var thiselement = 'h11';}
	  else if (i==20)  	{	var thiselement = 'h12';}
		
	  var p = document.getElementsByTagName(thiselement);
	  for(n=0; n<p.length; n++) {
		if(p[n].style.fontSize) {
		   var size = parseInt(p[n].style.fontSize.replace("px", ""));
		} else {
		   var size = 12;
		}
		p[n].style.fontSize = size+inc + 'px';
	   }
	}
}