DocBook

DocBook is an XML-based document encoding format which defines the semantic structure of a document. It originated as a technical writing format with the intention of producing documents in multiple formats from a single canonical source. As such, it was born with the tenet of decoupled rendering in mind.

Given that DocBook is XML format, it is intrinsically poor in terms authoring delight and has been eclipsed by markdown in recent years, despite its limitations compared to the former. Having said this, the DocBook specification is stable and well-documented and has an O’Reilly book published in 2010 which covers the 5th iteration of the specification.

Example

<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="this-is-a-level-1-heading">
  <title>This is a level 1 heading</title>
  <section xml:id="this-is-a-level-2-heading">
    <title>This is a level 2 heading</title>
    <para>
      This is text in <emphasis role="strong">bold</emphasis> and
      <emphasis>italics</emphasis>, and this is an external link to
      <link xlink:href="https://docops.guide">DocOps</link>. Now, some
      bullet points:
    </para>
    <itemizedlist spacing="compact">
      <listitem>
        <para>
          Bullet point 1
        </para>
      </listitem>
      <listitem>
        <para>
          Bullet point 2
        </para>
      </listitem>
      <listitem>
        <para>
          Bullet point 3
        </para>
      </listitem>
    </itemizedlist>
  </section>
</section>

Note that given DocBook’s semantic-oriented structure, a document is organized into the likes of chapters and sections rather than general ‘heading levels’.