Accessibility - Web Standards: Mastering HTML5, CSS3, and XML - page 349

These guidelines contain 61 success criteria . WCAG 2.0 conformance can be achieved by applying a series of
techniques [24]. Some of them are required to meet success criteria ( sufficient techniques ), while others are optional
only ( advisory techniques ). W3C claims that none of the accessibility techniques can be marked as required or
mandatory [25]. They are recommended only, and developers may choose to apply them.
In contrast to WCAG 1.0, which has three priority levels, WCAG 2.0 success criteria are organized into three levels
of conformance . The conformance requirements of all levels of WCAG 2.0 are summarized in the following sections.
Generally, the word must in the following sections—similar to the word use of the WCAG specifications published
by the W3C—corresponds to Level A conformance, which is the minimum level of accessibility in WCAG 2.0 (for
example, text alternative for nontext content). The word should correspond to Level AA conformance (for example,
captions for live synchronized media). The words may and can correspond to Level AAA conformance (for example,
optional sign-language interpreters).
The higher the accessibility conformance level, the more requirements or higher restrictions apply. For example,
Guideline 1.4 describes distinguishable color use and the requirements for separating foreground from background.
To meet level A, you should not rely on color alone for conveying information. Level AA has a stricter requirement and
prescribes a minimum contrast ratio of 4.5:1 (3:1 for large text), while Level AAA demands an even higher contrast
ratio of 7:1 (4.5:1 for large text).
Note that some requirements are general, while others are technology-specific and apply to a certain technology
only, such as Flash. Some techniques can be used either individually or in combination with a similar technique, such
as short and long descriptions, but the corresponding implementation requirements depend on the desired level of
accessibility.
Level AAA conformance is not recommended as a general policy, because there are content types that cannot
be published in a way that the document meets all AAA criteria due to the nature of the content or special technology
features and limitations. In other words, using certain content types might limit the maximum achievable level of
accessibility to WCAG 2.0 AA, while WCAG 2.0 AAA cannot be met if certain content types are present. For example,
section headings can contribute to AAA conformance; however, they cannot be added to all kinds of documents (for
example, a long letter [26]). Naturally, site structure or markup elements should be reorganized, added, or modified
when creating an accessible web site or redesigning a nonaccessible site to become accessible, but there is no reason
for a content author to modify the textual content of a web page just to make the site accessible or more accessible.
Another example is Flash, which had known accessibility barriers at the time the WCAG specifications were
introduced, and generally it was infeasible to provide Flash content and achieve level AAA conformance at the
same time.
Additionally, WCAG conformance can be limited to a conforming alternate version instead of the whole web site.
For example, if the complex design of a web site makes it infeasible to meet WCAG requirements, the site can still be
accessible by providing an alternate style sheet or, in the case of more sophisticated design, an alternate, accessible
version of each page within the site. Consequently, determining WCAG conformance requires deep site analysis and
cannot be judged by simply opening the home page [27].

■ in contrast to documents with tag soup and bad markup, well-structured, standard-compliant web sites with
properly written content provide a basic level of accessibility by default, which can be further extended to achieve the
desired (advanced) level of accessibility.

Note