实例代码：
<!doctype html>
<head>
<meta charset="utf-8">
<style type="text/css">
p.em3{font-size:3em;}
p.em15{font-size:1.5em;}
p.em1{font-size:1em;}
</style>
</head>
<html>
<body>
<h1>em字体大小实例</h1>
<p class="em3">这是3em的字体大小</p>
<p class="em15">这是1.5em的字体大小</p>
<p class="em1">这是1em的字体大小</p>
</body>
</html>
1em = 16px
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