var min=12;
var max=16;
var switchvar = true;
function increaseFontSize(){

   var p = document.getElementsByTagName('p');
   for(i=0;i<p.length;i++) {
      if(p[i].style.fontSize) {
         var s = parseInt(p[i].style.fontSize.replace("px",""));
      } else {
         var s = 12;
      }
      if(s != max && switchvar) {
         s += 1;
      }
  else {
  switchvar = false;
  s -= 1;
  if(s == 12)
  switchvar = true;
}
      p[i].style.fontSize = s+"px"
  
   }

}