Not everyone who accesses the Web can see the effects we've discussed in this book. Some 1.1 million people in the United States are blind, and, obviously, they have a very different experience of the Web than sighted persons.
Happily, CSS is not silent on the matter of non-visual access. CSS2 included the ability to apply styles in non-screen media. While the Web has seen most of its evolution take place on monitors that is to say, in a visual medium CSS2 can be used in non-visual media, assuming that the user agent has proper support.
The advantages of designing documents that are at once visually and non-visually usable should not be dismissed. If you can take one document and use different, medium-specific style sheets to restyle it for screen, print, and aural rendering, you can save yourself a whole lot of trouble. For example, you would have no need to link to "printer-friendly" versions of a page. Instead of creating totally different markup structures, one for screen and another for print, you can make your site more efficient by reusing the same document.
For that matter, it's possible to take a single HTML document that contains the outline of a slideshow and style it for easy reading on a screen, for clean and readable printouts, as a slideshow, and in a manner that a screen reader can translate. In the course of this chapter, we'll look at ways to do the latter three (since the rest of the book concerns itself with screen presentation).