var min=10;
var initial = 14
var max=20;
var step = 2;

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 = initial;
      }
      if(s!=max) {
         s += step;
      }
      p[i].style.fontSize = s+"px"
   }
}

function decreaseFontSize() {
   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 = initial;
      }
      if(s!=min) {
         s -= step;
      }
      p[i].style.fontSize = s+"px"
   }   
}