Headings provide structure for information on webpages. They mark the beginning of a section on a page, which makes information easier to find, comprehend and to act upon. Typically, they describe or at least hint at the text content that follows.
This functionality is even more important when it comes to people who use screen readers, such as blind or vision impaired users.
While it's common for content authors to use formatting techniques like setting headings in larger text sizes, different fonts and in bold, italics or underlined text to signify page structure, these visual cues are lost on the blind user.
However, screen readers can interpret page structure including headings, if it is correctly marked up. For this reason, using the correct HTML markup
<h1> or rather than increasing font size for example, is even more important when it comes to people who use screen readers.
Information, structure, and relationships conveyed through presentation can be programmatically determined or are available in text. (Level A)
Section headings are used to organise the content. (Level AAA)
These success criteria are about using elements, including headings, to organise and structure content on webpages, and making these structural elements accessible to as many people as possible.