A success criterion is the smallest testable unit of requirement in WCAG. Each criterion is written so that whether a page meets it can be determined objectively, and each is assigned to one of the levels A, AA or AAA. Claiming conformance at a level requires meeting all criteria at that level and below. The criteria set was updated with WCAG 2.2.
WCAG has a three-layer structure: four principles (POUR) at the top, guidelines beneath them and success criteria at the bottom. Principles and guidelines give direction but cannot be tested; the measurable layer is the criteria. Every criterion is numbered and written so it can be evaluated as true or false: criterion 1.4.3, for example, requires a contrast ratio of at least 4.5:1 for normal text, and a color pair either passes that threshold or does not. The levels form a triad: A contains the minimum criteria removing the most fundamental barriers; AA is the common target and the level most legal references point to; AAA holds the strictest criteria and may not be achievable for every type of content.
Conformance is cumulative: to claim 'AA conformant', all criteria at both Level A and Level AA must be met, and a single failing criterion drops the level claim for that page. WCAG 2.2 updated the criteria set, adding new criteria on top of previous versions; the 2.x versions are designed to be backwards compatible. In practice, audits run along the criteria list: automated scanning tests a portion of the criteria by machine, while the rest require manual testing with a keyboard and a screen reader.
Frequently asked questions
What do the WCAG success criterion levels (A, AA, AAA) mean?
Each success criterion is assigned to one of three levels. A contains the minimum requirements removing the most fundamental barriers. AA is the middle level and the common target that most legal references, such as the EAA, point to. AAA is the strictest level and may not be reachable for all content. The levels are cumulative: AA conformance also requires meeting every criterion at Level A.
If one criterion fails, does the site count as non-conformant with WCAG?
For the targeted level, yes: conformance requires meeting every criterion at that level, and a single violation drops the level claim for that page. That does not make the work worthless: every fixed violation removes a concrete barrier for real users. The right approach is to prioritize violations, fix them in order of severity and describe the conformance status honestly in your accessibility statement.
This content is for information only and is not legal advice.