var min=100;
var max=150;
function increaseFontSize() {
   var div = document.getElementsByTagName('div');
   for(i=0;i<div.length;i++) {
      if(div[i].style.fontSize) {
         var s = parseInt(div[i].style.fontSize.replace("%",""));
      } else {
         var s = 100;
      }
      if(s!=max) {
         s += 3;
      }
      div[i].style.fontSize = s+"%"
   }
}
function decreaseFontSize() {
   var div = document.getElementsByTagName('div');
   for(i=0;i<div.length;i++) {
      if(div[i].style.fontSize) {
         var s = parseInt(div[i].style.fontSize.replace("%",""));
      } else {
         var s = 100;
      }
      if(s!=min) {
         s -= 3;
      }
      div[i].style.fontSize = s+"%"
   }   
}