zoomIn and zoomout

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
 <head>
<script>
  var fontSize = 1;
function zoomIn() {
    fontSize += 0.1;
    document.body.style.fontSize = fontSize + "em";
}
function zoomOut() {
    fontSize -= 0.1;
    document.body.style.fontSize = fontSize + "em";
}
</script>
 </head>
 <body>
<input type="button" value="+" onClick="zoomIn()"/>
<input type="button" value="-" onClick="zoomOut()"/>
 </body>
</html>