Glossary · Accessibility term
Understandable
Understandable is the third of WCAG's four principles, and it splits in two the way its own sentence does. The information has to make sense, and the operation of the interface has to make sense. Three guidelines sit underneath it. Readable covers declaring the language of the page, and of foreign-language passages inside it that are not proper names, technical terms or words the surrounding text has absorbed. Predictable covers navigation staying consistent, and the several separately scoped rules about the page not changing under somebody without warning. Input assistance covers labels, error messages that explain themselves, and help getting a form right. Those three names are what let you place any rule whose number starts with a 3. And nothing conforms to a principle, so "we meet Understandable" says nothing on its own. The criteria underneath are what gets tested.
In practice
WCAG 2.2 added four criteria under this principle, second only to the five it added under Operable, and two of the four are Level A. Consistent help, meaning your help route stays in the same relative place across pages. And redundant entry, meaning information somebody already gave has to come back to them inside the same process, unless retyping it is essential, unless security demands it, or unless the earlier answer has expired. A checklist written before 2.2 is missing two Level A rules on this principle alone, which is the cheapest kind of gap to find and the easiest to overlook.
The other two additions both changed sign-in, one at AA and a stricter one at AAA. Accessible authentication says a login cannot depend on a cognitive function test. That puts puzzle CAPTCHAs, "type the third character of your password" and anything asking you to transcribe a code from memory under real pressure. Four ways out are written into the AA rule itself. Another method that avoids the test, a mechanism that helps the user through it, recognising objects, or identifying content the user provided. Password manager support and plain copy and paste are how the second of those usually gets built.
Readable delivers less at AA than its name promises. Unusual words, abbreviations, reading level and pronunciation all sit at AAA. At A and AA the guideline amounts to declaring the language of the page and of any foreign-language passage inside it. That is a much smaller job than "make text readable" sounds like, and it is why the checklist feels thin here.
It is the principle machines help with least, and the example makes the reason obvious. A machine can confirm an error message exists. Only a person can tell you it says "Invalid input" and leaves the user no wiser about which field or why.
Why it matters
This is where checkout lives. There is a Level AA criterion aimed squarely at transactional pages. A submission that creates a legal commitment, moves money, changes or deletes data the user controls, or submits test answers has to be reversible, checked before it goes through, or confirmed by the user first. Its failures are quieter than a missing alt attribute and considerably more expensive. W3C is also unusually honest about the ceiling here. The working group says in the standard itself that it ran into real difficulty writing testable criteria for cognitive, language and learning disabilities, and that work continues. That is a plain admission that the checklist runs out before the job does.
What it covers
Language, predictability and help with mistakes. A page can be perfectly perceivable and perfectly operable and still be impossible to follow, and nothing in the first two principles will catch that.
Where this shows up on the site
Related terms
- POURPOUR is the field's shorthand for WCAG's four principles, which are Perceivable, Operable, Understandable and Robust.
- Plain languagePlain language is writing that lets somebody find what they need, understand it the first time, and act on it.
- Error identificationError identification is the Level A rule that when your form rejects something, the page names the field that failed and says in text what went wrong.
- Form labelA form label is the visible text naming a field, tied to that field in code so software can tell which words belong to which box.
- Cognitive disabilityCognitive disability is a broad group of conditions affecting how people take information in, hold on to it, and act on it.
Knowing the word is the easy part.
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