Web Content Accessibility Guide (WCAG 2.0) is divided into three conformance levels (A, AA, and AAA).
They are organized 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. 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 most organizations will not be able to achieve that level (as the compromises on design will be too important).