var min=8;
var max=22;
function increaseFontSize() {
   var mydiv = document.getElementById('main_text_div');
      if(mydiv.style.fontSize) {
         var s = parseInt(mydiv.style.fontSize.replace("px",""));
      } else {
         var s = 12;
      }
      if(s!=max) {
         s += 1;
      }
      mydiv.style.fontSize = s+"px"
}
function decreaseFontSize() {
   var mydiv = document.getElementById('main_text_div');
      if(mydiv.style.fontSize) {
         var s = parseInt(mydiv.style.fontSize.replace("px",""));
      } else {
         var s = 12;
      }
      if(s!=min) {
         s -= 1;
      }
      mydiv.style.fontSize = s+"px"
}