Guideline 3. Use markup and style sheets and do so properly.
Section 508, Subpart D (Technical Standards) 1194.22
http://www.w3.org/TR/WCAG10/#gl-structure-presentation
“Mark up documents with the proper structural elements. Control presentation with style sheets rather than with presentation elements and attributes.”
Checkpoint 3.1 - When an appropriate markup language exists, use markup rather than images to convey information.
Checkpoint 3.2 - Create documents that validate to published formal grammars.
Checkpoint 3.3 - Use style sheets to control layout and presentation.
Checkpoint 3.4 - Use relative rather than absolute units in markup language attribute values and style sheet property values.
Checkpoint 3.5 - Use header elements to convey document structure and use them according to specification.
Checkpoint 3.6 - Mark up lists and list items properly.
Checkpoint 3.7 - Mark up quotations. Do not use quotation markup for formatting effects such as indentation.
Guideline 3 is a little longer than the other guidelines that we have covered thus far, but it can be summed up very easily: Design your website based upon structure, not upon presentation, and implementing CSS is the way you should do this. Avoid using elements and attributes in each individual page of your web site. Instead, use an external style sheet to maintain organization and style throughout your entire web site.
Using markup improperly (not according to specification) hinders accessibility. For example: Using a table for layout or a header to change the font size, makes it difficult for users with specialized software to understand the organization of the page or to navigate through it.
Lastly, when creating your external style sheet, avoid using absolute values (pt, cms). The page may look perfect when viewed on your own screen, but not everyone uses the same resolution or has the same size monitor as you. Use ems, exs, or percentages instead, which are relative values, these will render better on different displays and will help to maintain the layout and organization you had in mind when you designed your page.