WCAG 2.1 groups guidelines by their level of importance and their relative impact on accessibility. It is helpful to think of these levels as things that must, should, and could be done to eliminate potential barriers. The levels are Level A, Level AA, and Level AAA.
Level A guidelines address barriers that will prevent some groups from accessing web content. They MUST be addressed or the content will not be accessible to some people.
An example of a Level A barrier is an image that is not described in text (Success Criteria 1.1.1). There is little a person who is blind can do, without the help of another person, to determine what is being presented in an image when it is not described.
Level AA guidelines address barriers that may make it difficult to access web content, but it may still be possible through workarounds or added effort. They SHOULD be addressed, or the content will be more effortful to access than it needs to be.
An example of a Level AA barrier is keyboard focus that is not visible (Success Criteria 2.4.7). For someone with low vision who navigates web content with a keyboard, the inability to see when a link has focus, for instance, makes it difficult to know when to press Enter to activate the link. They may still be able to find their way to the link through trial and error, but a lot of unnecessary effort would be needed.
Level AAA guidelines address usability, more so than barriers. These items COULD be addressed to improve usability for everyone.
An example of a Level AAA usability issue is the lower-level high school reading–level requirement (Success Criteria 3.1.5). For instance, if someone reads in a second language, the use of simpler language, whenever possible, makes reading easier. The use of simpler language also improves accuracy when using automated translation tools. Even for typical readers reading in their first language, using simpler language is generally appreciated and easier to comprehend. For someone with a cognitive disability, simpler language will be easier to understand. In each case, however, the reading level does not prevent a person from accessing the content, but that content would be more usable if it were addressed.
Which Level Should Be the Goal?
Level AA is the generally accepted level of accessibility websites should aim to meet. If it is not possible to meet the requirements at this level, then Level A should be a temporary goal, while working toward Level AA over time. Very few websites will meet Level AAA requirements, and, in some cases, it may be counterproductive or undesirable to meet these guidelines. Take, for example, an online medical sciences book. If the Level AAA reading level guideline were followed, it would probably make the content unusable by the intended audience (medical students), if jargon and technical language is replaced with low level paraphrasing to meet this requirement. That said, most public websites that cater to a general audience should probably meet the lower-level high school reading–level requirement.
In addition to meeting Level AA requirements, websites can address some of the Level AAA guidelines, but meeting Level AAA should generally not be the goal.