Heading define the structure / outline / table of content of the document (page)
They are defined via:
They implicitly create a new section of the document
In a letter, the heading is one sentence that explains the reason of the letter
Because they defined the TOC, they should be in sequential order.
If this is not the case, the TOC will not have a hierarchical structure.
Any HTML analytics tool such Lighthouse will complain if this is not the case.
Grouped with a header element
<header>
<h1>My Title In the TOC</h1>
<p>My Subtitle Line 1</p>
<p>Editor’s Draft 9 May 2013</p>
</header>
The H(n) element applies a level-(n) heading syle to the contained text.
<h1>H1 - Generally the page title</h1>
<h2>H2 - Introduction</h2>
<h3>H3 - Prologue</h3>
<h4>H4 - Level-4 Heading</h4>
<h5>H5 - Level-5 Heading</h5>
<h6>H6 - Level-6 heading, smallest heading available</h6>
Using role=“heading to an element causes an AT (like a screen reader) to treat it as though it were a heading.
Ref:
You can add numbering to your heading via css. See heading numbering
Example:
section { counter-increment: section; }
h1::before { content: "Chapter " counter(section,lower-alpha) ": \A"; }
<section>
<h1>First</h1>
</section>
<section>
<h1>Second</h1>
</section>