You may have heard about the new outline algorithm that comes with HTML 5. A document’s outline is altered when it encounters a sectioning element like HEADER, ASIDE, ARTICLE, SECTION, and so on.
Roger Johansson has researched how the outline works and how it’s different from HTML4. If you’re writing HTML 5 and using these sectioning elements, it’s important to understand how this works. Poorly constructed markup can result in users being confused by the outline your markup generates.