Make available to the user an "outline" view of rendered content, composed of labels for important structural elements (e.g., heading text, table titles, form titles, and other labels that are part of the content).
Normative inclusions and exclusions
What constitutes a label is defined by each markup language specification. For example, in HTML, a heading (H1-H6) is a label for the section that follows it, a CAPTION is a label for a table, and the title attribute is a label for its element.
The user agent is not required to generate a label for an important element when no label is present in content. The user agent may generate a label when one is not present.