Design pages that function without style sheets
Under some circumstances, styles may not be part of a user’s experience of Web pages. Some users use browsers that do not support styles. Some users turn off style sheets or apply a custom style sheet. Nonvisual users do not access visual styles. These users may encounter difficulties if designers rely on styles, for example, to divide functional areas of a page or to group related elements. To support users who do not access styles, structured documents must be functional, comprehensible, and usable without the formatting supplied via style sheets.
For pages to function without style sheet formatting, content in the HTML document must be logical when read or viewed in sequence. The sequence of page elements must follow a logical order—for example, header, navigation, content, and footer. Moreover, the content belonging in each section must be contained within each element.
In general, pages that are structured using basic HTML tags will work best when users access them without styles, or with user-defined styles. Pages that are designed using nonstructural elements do not adapt as well as pages designed using structural elements, such as paragraphs and lists. Browser or user-defined styles and screen readers cannot account for custom elements—
<div id="footer">—and will not have the means to differentiate these elements. However, if elements are designed using structural markup—
<ul id="footer">—software can differentiate these elements by accessing the structural tags.