Use markup according to specification. For example, use header elements to convey document structure. Mark up lists and list items properly. Mark up quotations. Do not use quotation markup for formatting effects such as indentation.
Header elements in Web pages are meant to function as they do in written literature. They mark chapters, sub-sections and so forth. Many people navigate or skim through documents by reading the headings to get a feel for the structure and an overview of the content and scope of a document. Misusing the header element in an html document can cause a misinterpretation in the way you wish to present your information.
Also, some screen readers will read content assigned as a header in a different tone of voice to other content on the page, providing users with important clues about document structure. Misusing heading attributes will therefore misinform and confuse screen reader users. (INDA)
<ol> <li>This is the first item.</li> <li>This is the second item.</li> <li>This is the third item.</li> </ol>
Screen reader users will refer to the numbers in an ordered list for navigation and context. If the numbers do not relate to a clear structural hierarchy, users will be easily confused.
"The power of the Web is in its universality. Access by everyone regardless of disability is an essential aspect."
-- Tim Berners-Lee, W3C Director and inventor of the World Wide Web
<blockquote> "The power of the Web is in its universality. Access by everyone regardless of disability is an essential aspect." <br /> -- Tim Berners-Lee, <acronym title="World Wide Web Consortium">W3C</acronym> Director and inventor of the World Wide Web </blockquote>
Using header elements - WAI recommended technique.
View WAI checkpoint 3.5 - Use header elements to convey document structure and use them according to specification.