Preview:
<html>
<head>
    <style type="text/css">
html, body {
    height: 100%;
    width: 100%;
    padding: 0;
    margin: 0;
}

body {
    display: flex;
    flex-direction: column;
}

header {
    height: 75px;
}

main {
    flex: auto;
    background-color: #ccc;
}

footer {
    height: 25px;
}
    </style>
</head>
<body>
    <header>header</header>
    <main>main</main>
    <footer>footer</footer>
</body>
</html>
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