Web Design in a Nutshell, 3rd Edition

Use semantic markup

It is also important to mark up elements in your document semantically, that is, in a way that is descriptive and meaningful. There is a renewed emphasis on choosing elements to appropriately describe the content and not purely for their presentational effects in the browser. Techniques that once were common, such as marking up content as a list just to get the text to indent, are now deemed completely unacceptable.

Semantic markup is not the same as standards compliance. It is possible to create a document out of font, br, and i elements that validates entirely, but that does zilch for making the content meaningful. A semantically marked up document ensures accessibility in the widest range of browsing environments, from desktop computers to cell phones to screen readers. It also allows nonhuman readers, such as search engine indexing functions, to correctly parse your content and make decisions about how to handle it.