Use the <h1> to <h6> tags, in order to mark the headers of a document correctly. Start with <h1> on the first level.
<h1>
<h6>
<h1>Header A</h1> <h2> Header A.1</h2> <h2> Header A.2</h2> <h3> Header A.2.1</h3> <h1> Header B</h1>