You can go beyond 6 with ARIA, e.g. role="heading" aria-level="7". I don't know what the limit is. I think it's challenging to find distinct visual styles for many heading levels.

They tried to make the the explicit number irrelevant and infer the level by the number of nested sections, so instead of:

    <article>
      <h1>Title</h1>
      <section>
        <h2>Section</h2>
        <section>
          <h3>Sub Section</h3>
          <section>
            <h4>Sub Sub Section</h4>
          </section>
        </section>
      </section>
    </article>

You could simply use <h2> repeatedly and keep the semantic structure of the former.

    <article>
      <h1>Title</h1>
      <section>
        <h2>Section</h2>
        <section>
          <h2>Sub Section</h2>
          <section>
            <h2>Sub Sub Section</h2>
          </section>
        </section>
      </section>
    </article>

However this was never implemented by any browser or screen reader and was finally dropped from the standard.

http://html5doctor.com/computer-says-no-to-html5-document-ou...