Live Demo: Defining Font Size Using REM

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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Defining Font Size Using REM</title>
<style>  
    html {
        font-size: 62.5%;    /* font-size 1em = 10px */
    }
    body {
        font-size: 1.6rem;    /* 1.6rem = 16px */
    }
    p {
        font-size: 1.4rem;    /* 1.4rem = 14px */
    }
    p span {
        font-size: 2rem;    /* 2rem = 20px (not 28px) */
    }
</style>
</head>
<body>
    <h1>This is a heading</h1>
    <p>This is a <span>paragraph</span>.</p>
</body>
</html>                            