Documents shall be organized so they are readable without requiring an associated style sheet.

Why this is important: Many low vision visitors will disable the page author's styles, and may substitute a custom style sheet of their own. If styles are used to substitute for semantic markup, low-vision and text-only readers will be unable to understand the structure of the page. (Example: large bold text used instead of a heading.) Elements that are hidden from sighted users will also be hidden from low-vision users who may need to see them; badly hidden element may be inaccessible to all users.