Web Design in a Nutshell, 3rd Edition

Keep presentation separate from structure

Use style sheets to control presentation. Keeping all presentation information in a separate style sheet document makes it easier to redesign or repurpose content. In terms of markup, this means avoiding presentational (X)HTML elements and attributes that are still hanging around in the Recommendations (such as b for bold text), and using an appropriate semantic alternative (e.g., strong) with a style sheet rule.