var px = new Array(9,10,11,12,13,14,15,16,17,18,19,20,21);  
    var size = 3;
function do_change_down(elemID) {
        
    if (size > 0) {
        size--;
        document.getElementById(elemID).style.fontSize = px[size]+'px';
    }
    
}
function do_change_up(elemID) {
    
    if (size < 12) {
        size++;
        document.getElementById(elemID).style.fontSize = px[size]+"px";
    }
    
}