function fontChanger(operation, div_name, default_size, default_line) {
  // Setting up array of tags, use as many as you want.
  var tagList = new Array('p', 'h1', 'h2', 'h3', 'h4', 'h5', 'th', 'td', 'li');

  // Looping through array of tags
  for(var i = 0; i < tagList.length; i++) {
    var allTags = document.getElementById(div_name).getElementsByTagName(tagList[i]);
    for(var j = 0; j < allTags.length; j++) {
      // If a font size is set, use it, else use "default_size"
      if(allTags[j].style.fontSize) {
        var size = parseInt(allTags[j].style.fontSize.replace('px', ''));
        var line = parseInt(allTags[j].style.lineHeight.replace('px', ''));
      } else {
        var size = default_size;
        var line = default_line;
      }

      switch(operation) {
        case 0:
          size += 1;
          line += 1;
          break;

        case 1:
          size -= 1;
          line -= 1;
          break;

        case 2:
          size = default_size;
          line = default_line;
          break;

        default:
          break;
      }

      // Finally, re-write style with new font size
      allTags[j].style.fontSize = size + 'px';
      allTags[j].style.lineHeight = line + 'px';
    }
  }
}