Live Demo: Defining Font Size Using EM

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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Defining Font Size Using EM</title>
<style>  
    h1 {
        font-size: 2em;    /* 32px/16px=2em */
    }
    p {
        font-size: 0.875em;    /* 14px/16px=0.875em */
    }
</style>
</head>
<body>
    <h1>This is a heading</h1>
    <p>This is a <span>paragraph</span>.</p>
</body>
</html>                            