var min=8;
var max=18;
function increaseFontSize() {
   var p = document.getElementsByTagName('p');
   incSize(p);
   
}
function incSize(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 = 13;
      }
      if(s!=max) {
         s += 1;
      }
      p[i].style.fontSize = s+"px"
   }
}

function decreaseFontSize() {
   var p = document.getElementsByTagName('p');
   decSize(p);
   
}
function decSize(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!=min) {
         s -= 1;
      }
      p[i].style.fontSize = s+"px"
   }
}