function changeFontsize(increment) {
  if (document.getElementsByTagName) {
    var getElement = document.getElementsByTagName("p");
    var eachElement, currentFontSize, currFontSize, fontIncrease, newFontSize;
    for (i=0; i<getElement.length; i++) {
      eachElement = getElement[i];
      if (document.defaultView)   // Mozilla
        currentFontSize = document.defaultView.getComputedStyle(eachElement, null).getPropertyValue("font-size");
      else if (eachElement.currentStyle)   // IE
        currentFontSize = eachElement.currentStyle.fontSize;
      currFontSize = parseInt(currentFontSize, 10);
      fontIncrease = parseInt(increment, 10);
      newFontSize = currFontSize + fontIncrease;
      eachElement.style.fontSize = newFontSize + "px";
    }
  }
}