var pixelArray =  new Array('9', '10', '11','12', '13','14','16','18','20');
var initSize = 2;

function changefontSize(id,inc,unit) {
	if (!document.getElementById) 
		return;
	var size = initSize;
		size += inc;
	if (size < 0 ) {
		size = 3;
}
	if (size > 8 ) {
		size = 3;
}
		initSize = size;
        document.getElementById(id).style.fontSize = (unit=='px') ? pixelArray[size]+unit: emArray[size]+unit;
}