It might not working because you have multiple head, html and body.
It should be :
index.php
<html>
<head>
<title>Armenian Genocide</title>
<link rel="stylesheet" type="text/css" href="stylesheet.css">
<body>
<?php include ('header.php') ?>
<div id='content'>
</div>
<?php include ('footer.php') ?>
</body>
</html>
header.php
<div id='head'>
<h1>HEADER</h1>
</div>
footer.php
<div id='footer'>
<h1>FOOTER</h1>
</div>