HTML Unleashed PRE. Creating Widely Accessible Web Pages: The Two Principles

he first accessibility principle is presenting your information in alternative modalities---that is, using as many different media and different input/output techniques as possible.  For example, the audio channel may work for the blind, while the visual channel would duplicate this information for the deaf.  This principle is especially important for the "bottlenecks," that is, the elements that are critical for delivering your message, for understanding of what follows, or for navigating to other parts of the site.

In practice, the alternative modalities principle most often amounts to the imperative of providing textual parallels for all non-textual information.  This is because plain text is the most natural informational medium for the computer; it is easy to display with any sort of computer interface, easy to index and process automatically, and relatively easy to convert to speech.  By presenting information as text, we take advantage of the "inherent accessibility" of this format and may not be concerned any more about alternative modalities.

However, a page doesn't need to be text-only to be accessible.  The principle of alternative modalities and the provisions of HTML allow you to produce pages that are both presentationally rich and accessible at the same time.  If you need an audio clip, accompany it with a text transcript; if you want an image (as you most probably do), give it an alt text conveying as much of the image's information as possible (for more on this, see "Graphics" later in the chapter); and if you feel an urge to employ a Java applet or Shockwave object, make sure it is not critically necessary for navigating your pages or reading the text (and if that's not the case, provide an alternative access mechanism for the same information).

In other words, an accessible web page may be multi-modal (i.e. may use various input/output techniques) provided that all its critical parts are sufficiently alt-modal (i.e. no important part of its content is locked within one modality).  The art of creating alt-modal web pages while keeping them efficient, rich, and elegant enough to be competitive in today's Web might seem tricky, but actually it's only a matter of obeying some simple and obvious guidelines (most of which are summarized in this chapter).

The second fundamental principle of accessibility is using logical, not presentational, markup for your content.  (You may be familiar with these concepts from Chapter 4, "The HTML Document Type Definition" that deals with the SGML roots of HTML.)  Logical markup means that for every element of your page, you should use HTML tags to describe what it is rather than how it's supposed to look (or sound, or print, or whatever).

The importance of logical markup reaches far beyond the problem of accessibility for the disabled; in fact, it is the best method to ensure the general accessibility, processibility, and, in the bottom line, longevity of your information.  When the computer (not only your human reader) is aware of the logical structure of your data, the chances of its being able to successfully convert it into another format or media are very much improved.  And accessibility implies, for the most part, exactly this: the ease of transforming the document into another media or presentation mode, in order to be able to communicate it to people with disabilities.

One example is the controversy of the I tag versus the EM tag.  The former one, being a tool of presentational markup, directs the browser to switch to an italic font and is thereby limited to visual presentation mode only.  A speech browser has no "italic font" and is therefore likely to ignore this piece of markup altogether.  Conversely, EM is a logical tag meaning emphasis; while a visual browser may interpret this element by switching to an italic font, a speech browser may read it aloud with a different tone of voice ("acoustic emphasis").  An additional advantage is that EM allows distinguishing italics for emphasis from other uses of italics, such as citations (to be marked up by the CITE logical tag).

Naturally, web authors often need tools to adjust presentational aspects of their documents as well---for instance, to control font face, or size, or voice pitch.  HTML's answer to this demand is style sheets, an external mechanism capable of specifying a multitude of presentational parameters in a flexible manner (see https://www.w3.org/TR/REC-CSS1).  The current specification of HTML style sheets supports only visual presentation parameters, but there exists a proposed extension for handling aural (speech) parameters as well (https://www.w3.org/TR/WD-acss).