WCAG 2.0 is divided into three conformance levels (A-AA-AAA) because the success criteria are organised based on the impact they have on design or visual presentation of the pages. The higher the level, the more restraining it becomes on design.
For example, let's take guideline 1.4 (Distinguishable), which is about making it easier for users to see and hear content including separating foreground from background.
- Success Criterion 1.4.1 is about the
Use of Color. The rule goes like
this: Color is not used as the only
visual means of conveying
information, indicating an action,
prompting a response, or
distinguishing a visual element.
(Level A)
- Success Criterion 1.4.3 is about
Contrast (Minimum). The rule goes
like this: The visual presentation of
text and images of text has a
contrast ratio of at least 4.5:1,
except for the following: (Level AA)
- And finally, success criterion 1.4.6
is about Contrast (Enhanced). And the
rule goes like this: The visual
presentation of text and images of
text has a contrast ratio of at least
7:1, except for the following: (Level
AAA)
As you can see, the more higher we get, the more demanding it becomes and the more pressure it put on how things can be presented in the web page.
As a rule of thumb, success criteria from level A should be invisible or barely noticeable to the interface. On the other hand, level AAA will have such a high impact on design, that even the W3C claims that most organizations will not be able to achieve that level (as the compromises on design will be too important):
Source: http://www.w3.org/TR/WCAG20/#conformance-reqs (WCAG 2.0) - from the W3C
"Note 2: It is not recommended that Level AAA conformance be required as a general policy for entire sites because it is not possible to satisfy all Level AAA Success Criteria for some content."
Working towards compliance to one conformance level or another in any given project shouldn't have anything to do with what the client pays or doesn't pay. It should always be about making everything you can (within the boundaries of resources and budget of course) to ensure that all users, regardless of disabilities or limitations, can enjoy the content that is provided to the public. And this is especially true when it comes to government content.
