Conformance to a standard means you meet the requirements of the standard. In WCAG 2.0, the requirements are the Success Criteria. To conform to WCAG 2.0, you need to satisfy the Success Criteria, that is, there is no content which violates the Success Criteria.
Note: This means that if there is no content to which a success criterion applies, the success criterion is satisfied.
The Success Criteria were assigned to one of the three levels of conformance by the working group after taking into consideration a wide range of interacting issues. Some common factors include:
Is the Success Criterion essential - when it isn't met, even assistive technology can't bridge the gap within its API to make the content accessible.
Is it possible to satisfy the Success Criterion for all Websites and type of content.
Does the Success Criterion require skills that could reasonably be achieved by the content creators (knowledge and skill to meet the Success Criteria could be acquired in a week's training or less).
Does the Success Criterion impose limits on the "look & feel" and/or function of the Web page (limits on function, presentation, freedom of expression, design, or aesthetic).
Are there no workarounds if the Success Criterion is not met.
WCAG 2.0 is divided into three conformance levels (A-AA-AAA) because the success criteria 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.
Example 1: Contrast Ratio
AA = Contrast ratio of at least 4.5:1 between background and foreground. 3:1 for large text (over 18 point or 14 point bold).
AAA = Contrast ratio of at least 7:1 between background and foreground. 4.5:1 for large text (over 18 point or 14 point bold).
Example 2: Distinguishable
Focuses on making it easier for users to see and hear content including separating foreground from background. 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 a high impact on design and even the W3C indicates most organizations will not achieve that level with their current designs.
Success Criterion 1.4.1 - Use of Color
Rule: 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 - Contrast
Rule: The visual presentation of text and images of text has a contrast ratio of at least 4.5:1 (Level AA).
Success Criterion 1.4.6 - Contrast (Enhanced)
Rule: The visual presentation of text and images of text has a contrast ratio of at least 7:1 (Level AAA).
Source: Karl Groves
The first thing to keep in mind is that all WCAG Success Criteria are important.
Level A Success Criteria are those which will have a high impact on a broad array of user populations. In other words, they (usually) don't focus on one type of disability only. They will also have the lowest impact on the presentation logic and business logic of the site. Finally, implementation of these requirements will typically be the easiest.
Level AA Success Criteria will also have a high impact for users. Sometimes only specific user populations will be impacted, but the impact is important. Adherence to these Success Criteria may impose changes to a system's presentation logic or business logic.
Level AAA Success Criteria are often focused on improvements for specific user populations. They may be difficult or expensive to adhere to, depending on platform limitations. The benefit -to-cost ratio may be low enough to deprioritize these items.