var min=8;
var max=18;

function changeFontSize(change)
{
  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 ((change == 1 && s != max) || (change == -1 && s != min))
     s += change;

    p[i].style.fontSize = s+"px";
  }
}