The first level of structure to add to a page is headings.
Headings are used to break up the content into a hierarchical structure much like an outline.
Where sighted users can skim a page quickly by reading the visual headlines, a screen reader user can also skim a page by quickly traversing the headings.
To semantically define headings simply use heading tags to label the beginning of each section.
Recommendations
For all pages make sure heading level should be in hierarchical structured.
E.g. <h1><h2><h3>...
Also, to make any element a heading or to change a level of heading for screen reader, provide role="heading" aria-level="level for heading (1,2,3 etc)" to the element or use <h1>, <h2>, <h3>... instead.
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Recommendations