font-size: 150%; (50% larger)
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
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<html xmlns='http://www.w3.org/1999/xhtml' xml:lang='en'>
<head>
<title>font-size</title>
<style type='text/css'>
body {
font: 16px sans-serif;
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span {
background: mistyrose;
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</head>
<body>
<p>
The font-size property supports a variety of methods for
specifying a font size. For example, there
are seven different absolute size keywords, which set the
font size relative to the user's font size preference.
</p>
<ul>
<li style='font-size: xx-small;'>xx-small</li>
<li style='font-size: x-small;'>x-small</li>
<li style='font-size: small;'>small</li>
<li style='font-size: medium;'>medium</li>
<li style='font-size: large;'>large</li>
<li style='font-size: x-large;'>x-large</li>
<li style='font-size: xx-large;'>xx-large</li>
</ul>
<p>
You can also make fonts
<span style='font-size: larger;'>larger</span> or
<span style='font-size: smaller;'>smaller</span> by way
of the <span style='font-size: larger;'>larger</span> or
<span style='font-size: smaller;'>smaller</span> keywords.
</p>
<p>
You can make fonts
<span style='font-size: 150%;'>50% larger</span>
or <span style='font-size: 75%;'>25% smaller</span> by way
of percentages.
</p>
<p>
You can even make a font
<span style='font-size: 1.5em;'>50% larger</span>
or <span style='font-size: 0.75em;'>25% smaller</span> by way
of em units.
</p>
</body>
</html>