<!-- Font Size Script Paragraphes et Span -->
var min=8;
var max=18;
function increaseFontSize() {
   var p = document.getElementsByTagName('p');
   for(i=0;i<p.length;i++) {
      if(p[i].style.fontSize && p[i].style.lineHeight) {
         var s = parseInt(p[i].style.fontSize.replace("px",""));
		 var h = parseInt(p[i].style.lineHeight.replace("px",""));
      } else {
         var s = 14;
		 var h = 15;
      }
      if(s!=max) {
         s += 1;
		 h += 1;
      }
      p[i].style.fontSize = s+"px";
	  p[i].style.lineHeight = h+"px";
   }
   
var sp = document.getElementsByTagName('span');   
   for(i=0;i<sp.length;i++) {
      if(sp[i].style.fontSize && sp[i].style.lineHeight) {
         var spany = parseInt(sp[i].style.fontSize.replace("px",""));
		 var spanh = parseInt(sp[i].style.lineHeight.replace("px",""));
      } else {
         var spany = 12;
		 var spanh = 13;
      }
      if(spany!=max) {
         spany += 1;
		 spanh += 1;
      }
      sp[i].style.fontSize = spany+"px";
	  sp[i].style.lineHeight = spanh+"px";
   } 
}
/* ------------------------------------------------------------------------------- */
function decreaseFontSize() {
  var p = document.getElementsByTagName('p'); 
   for(i=0;i<p.length;i++) {
      if(p[i].style.fontSize && p[i].style.lineHeight) {
         var s = parseInt(p[i].style.fontSize.replace("px",""));
		 var h = parseInt(p[i].style.lineHeight.replace("px",""));
      } else {
         var s = 12;
		 var h = 13;
      }
      if(s!=min) {
         s -= 1;
		 h -= 1;
      }
      p[i].style.fontSize = s+"px";
	  p[i].style.lineHeight = h+"px";
   }
  var sp = document.getElementsByTagName('span');   
   for(i=0;i<sp.length;i++) {
      if(sp[i].style.fontSize && sp[i].style.lineHeight) {
         var spany = parseInt(sp[i].style.fontSize.replace("px",""));
		 var spanh = parseInt(sp[i].style.lineHeight.replace("px",""));
      } else {
         var spany = 13;
		 var spanh = 14;
      }
      if(spany!=min) {
         spany -= 1;
		 spanh -= 1;
      }
      sp[i].style.fontSize = spany+"px";
	  sp[i].style.lineHeight = spanh+"px"
   }   
}
<!-- Insert lines -->
// <a href="javascript:decreaseFontSize();">-</a> 
// <a href="javascript:increaseFontSize();">+</a>
 
