Section 508 - (d)
Documents shall be organized so they are readable without requiring an associated style sheet.
Explanation – Structure, Style & Read Order
Style sheets may be used for color, indentation and other presentation effects, but the document is still understandable when the style sheet is turned off.
Sematic markup is used to define the structure of the document - headings, paragraphs, lists, quotes, etc.
Assistive technology reads the underlying source code. The document's "read order" should not be confusing. Style sheets and layout tables may present the page in an order different from the read order.