[Details] Avoid any presentational markup and keep all the layout features in well-defined CSS style sheets. [Rationale] Linked style sheets allow presentation to be decoupled completely from the page structure of your site, and lets you quickly change its layout and rendering. It also guarantees that everybody can adjust the rendering to their needs and tastes. [Test tool] The CSS Validator detects any non-compliant use of the CSS technologies. [Requirement] 
Use extensively valid CSS