What's the difference?
What WCAG Levels are there?
The WCAG guidelines have three levels of conformance that you can meet; A, AA and AAA, and each is progressively harder.
Level A is a subset of Level AA. There are 25 criteria that you have to meet to reach Level A.
To reach Level AA you have to meet 13 more criteria in addition to the 25 for Level A.
Level AAA is the icing on the cake with 23 more criteria.
What's the difference between the three WCAG Levels?
The success criteria at Level A are designed to be the easiest to meet, without much impact on the website design or structure.
For example, at Level A you aren’t allowed to identify something by color alone, like “Press the green button to proceed”.
Your site probably passes this example already unless it has a load of complex data charts on it that have colored legends.
Level AA requires a bit more commitment. Continuing with the theme of color, you have to ensure that all the text meets color contrast requirements. The requirement differs somewhat based on the size of the text, but it is actually pretty strict.
In my experience, the majority of websites don’t meet this criterion. You need to review the contrast across the whole site, and revise colors where needed, even if they are your brand colors. (Logos are excluded from the rule, however).
At Level AAA, the requirement is taken further with an even more strict color contrast requirement for text. Essentially you can only use very dark colors on a very light background and vice versa. Almost all colored text fails.
Your average website doesn’t target Level AAA conformance, that level tends to be for pretty specialist sites as the criteria are really strict. That's not to say that they aren't helpful and it's a good idea to at least understand them as there might be examples that are easy for your site to meet.
In addition to the ease of implementation, another reason for a criterion to be at a certain level is the number of users it benefits.
If your website meets Level A, the majority of users will be able to use the site. Each additional level, in fact each criterion met makes your site accessible to more and more people.
Can I partially meet a WCAG level?
No. To state that your website is Level AA compliant, that means it has to meet every one of the Level A and the Level AA guidelines.
Don't use this as a reason not to try though. If meeting one of the criteria is really, really hard for your team, a website that meets 24/25 criteria for Level A will be so much more accessible than a website that meets none.
What level should I aim for?
Level AA is the level that most development teams are aiming to meet. This is the level that is legally required for certain sites and is the one that is typically referred to when you're tasked with "making a website accessible".
If you’re amending an existing site to be accessible, I’d recommend tackling the Level A criteria first and then moving on to Level AA.