var minTextSize = 80; // minimum font size: 80% var maxTextSize = 150; // maximum font size: 150% // derived from http://www.white-hat-web-design.co.uk/articles/js-fontsize.php function changeFontSize(changeValue) { var bodySize = document.getElementsByTagName('body'); if (bodySize[0].style.fontSize) { var newSize = parseInt(bodySize[0].style.fontSize.replace("%","")); // if the font size for the tag has been defined, extract the numerical value } else { var newSize = 100; // if the font size for the tag is NOT defined, set to 100 } switch (changeValue) { case "increase": if (newSize != maxTextSize) newSize += 10; break; case "decrease": if (newSize != minTextSize) newSize -= 10; break; default: newSize = 100; break; } bodySize[0].style.fontSize = newSize + "%"; }