Definition of HTML "sectioning content" and "section outline"

Sectioning content defines the scope of heading elements and footer elements. The tags for HTML elements that create sectioning content are:

• <article>
• <aside>
• <nav>
• <section>

These tags explicitly create a section. When they are not coded, the following tags will implicitly create a section:

• <hgroup>
• any of the <hn> heading tags without a parent <hgroup>

A section outline (not to be confused with the CSS outline property) is a tree representation of the sections starting with a sectioning root tag and including the sections explicitly or implicitly created within the scope of the outline. No more than one heading for each section is included in the outline.