The UK Government accessibility requirements say that services must achieve WCAG 2.1 level AA or higher to be compliant with the law. WCAG, or Web Content Accessibility Guidelines, are a set of internationally recognised set of recommendations for improving web accessibility.
There are three layers of conformance – A, AA and AAA, with A being the lowest and AAA the highest. Each is nested inside the other so AA conformance includes A and AA criteria and AAA conformance includes A, AA and AAA criteria. Each level has a detailed list of success criteria. When a website is assessed for accessibility conformance each of these criteria are tested. It may be the case that all the A and AA criteria are met, in which case a website accessibility statement can claim “This website is fully compliant with the Web Content Accessibility Guidelines version 2.1 AA standard.”
However, what if most (more than half) of the criteria are met but some are not? Then the statement can claim “This website is partially compliant with the Web Content Accessibility Guidelines version 2.1 AA standard, due to the non-compliances listed below.” and the statement goes on to list or link to a report of non-compliances against the success criteria.
It may be that it is simply not possible for an organisation to fix all the issues to make a site fully compliant with WCAG2 2.1 AA in which case they can claim ‘disproportionate burden‘ in other words, the impact of fully meeting the requirements is too much for the organisation to reasonably cope with. If they do this they must carry out an assessment.
WCAG produces a handy reference site for the A, AA and AAA criteria.