On the surface, a structured document looks no different from one that uses font size and other visual formatting to distinguish elements such as headings. When it comes to functionality, however, structure gives power and utility to the web. Take, for example, the heading of this section. Structural markup is visually identifiable as a heading because it is bold and sits directly above plain text. But software cannot infer that it is the primary subheading of this section because means nothing more than bold. If the chapter title is marked as Structural markup, software knows that the page is about structural markup, and that in turn facilitates all kinds of functions, such as returning the page on searches for structural markup or adding the page to compilations about structural markup.
When marking up text, think about what each text element is and not what it should look like. Tag each element with the appropriate html structural tag, and then use css to manage its visual properties (see chapter 8, Semantic emphasis).