Percentage font sizes apply a font size relative to the parent element's font size.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns='http://www.w3.org/1999/xhtml' xml:lang='en'>
<head>
<title>font-size</title>
<style rel='stylesheet' type='text/css'>
span {
background: mistyrose;
font-size: 150%;
}
span span {
background: pink;
font-size: 125%;
}
</style>
</head>
<body>
<p>
<span>
This font size is 50% bigger than that of the
<p> element.<br />
<span>
This font is 25% bigger than the font used
for the parent <span> element.
</span>
</span>
</p>
</body>
</html>