<!DOCTYPE html>
<html>
<head>
<title>Title of the document</title>
</head>
<body>
<h1>First-level heading</h1>
<h2>Second-level heading</h2>
<h3>Third-level heading</h3>
<h4>Fourth-level heading</h4>
<h5>Fifth-level heading</h5>
<h6>Sixth-level heading</h6>
</body>
</html>