There are some sites on the web that allow users to change their font size. This allows their site to be read by users who may have a hard time reading the default font size, and their site is more user friendly because of this feature.
<script language="JavaScript" type="text/javascript">
function fontSize(size){
document.getElementById("body").style.fontSize = size
}
</script>
<body id="body">
Select a font size:
<select name=fontChanger onchange="fontSize(this.value)">
<option value="8">8</option>
<option value="10">10</option>
<option value="12">12</option>
<option value="14">14</option>
<option value="16">16</option>
<option value="18">18</option>
<option value="20">20</option>
</select>
<div id="comments">
This is some text, which can have the size changed.
</div>