WCAG 2.2 · Principle 3 of 4
Understandable
Will people follow what your interface says and predict what it will do?
A page can be perceivable and operable and still be unusable. Understandable covers the rest of it. Can a screen reader pronounce your page? It can only do that once it has been told what language the page is in. Does the interface behave the same way twice? And when somebody gets a field wrong, does the form say what went wrong, say how to fix it, and keep the work they would already done?
An audit tests 13 rules here across three guidelines, fewer than under either Perceivable or Operable, and WCAG 2.2 changed this principle more than any other. Both of the two Level A criteria WCAG 2.2 added landed here. One keeps help in the same place across pages that share a purpose, and one stops a form demanding information you already gave. Neither is optional at Level A. Almost nothing under this principle can be cleared by a scanner either, because most of its criteria have no automated test rule at all. And Understandable is a principle rather than a rule, so an audit marks the 13 criteria under it and never the principle itself.
- Rules
- 13
- Level A
- 7
- Level AA
- 6
- Human testing only
- 8
All 13 Understandable rules, by guideline
3.1 Readable· The page has to say what language it is in, and mark where that changes.
- 3.1.1Level ALanguage of PageEvery page has to declare its main language in a way software can read, and on the web that means one attribute on the html element.Scanner-detectable4 fixes
- 3.1.2Level AALanguage of PartsWhen a passage switches language, that passage needs its own lang attribute, so the screen reader switches pronunciation for it and switches back afterwards.Partly scannable2 fixes
3.2 Predictable· The interface has to behave the same way twice, and never change on its own.
- 3.2.1Level AOn FocusLanding focus on something must never change the context on its own.Human testing2 fixes
- 3.2.2Level AOn InputChanging a setting must not change the context on its own, unless you told the user it would before they touched the control.Human testing8 fixes
- 3.2.3Level AAConsistent NavigationAnything that helps people move around your site has to keep the same relative order on every page that repeats it.Human testing4 fixes
- 3.2.4Level AAConsistent IdentificationThe same function has to be identified the same way across a set of pages, so a control that adds an item to the cart cannot be Add to Cart in one place and Buy in another.Human testing2 fixes
- 3.2.6Level ANew in 2.2Consistent HelpThis one is new in WCAG 2.2, and it is conditional.Human testing1 fixes
3.3 Input Assistance· Forms have to help people avoid mistakes and recover from the ones they make.
- 3.3.1Level AError IdentificationWhen a form rejects something, the page has to say which field is wrong and what is wrong with it, in text.Partly scannable13 fixes
- 3.3.2Level ALabels or InstructionsEvery field that takes input needs a label or an instruction, and the rule means every field rather than only the required ones.Partly scannable17 fixes
- 3.3.3Level AAError SuggestionWhen your code knows what would fix an error, it has to say so.Human testing9 fixes
- 3.3.4Level AAError Prevention (Legal, Financial, Data)Some pages do things that cannot be taken back, and this rule says at least one safeguard has to stand in front of them.Human testing7 fixes
- 3.3.7Level ANew in 2.2Redundant EntryNew in WCAG 2.2, and it says you cannot make somebody type the same thing twice inside one process.Human testing1 fixes
- 3.3.8Level AANew in 2.2Accessible Authentication (Minimum)New in WCAG 2.2.Partly scannable3 fixes
What Understandable actually covers
Whether people can follow the content and predict what the interface will do next. A page can be perceivable and operable and still be impossible to get through.
Three guidelines carry it. Readable covers the language declaration a screen reader needs before it can pronounce anything. Predictable covers consistency and changes nobody asked for. Input assistance covers labels, errors, and getting somebody back on their feet after a mistake.
The gap between what an audit tests and what the principle contains is sharper here than anywhere else. Two thirds of Readable sits at Level AAA, and the four rules up there are the ones about plain writing, which is what most people expect the whole principle to be about.
What fails most here
An error message that says something is wrong without saying what, or how to fix it.
Validation messages get written by whoever coded the rule, so they describe the rule. Invalid input announces the rule and helps nobody move forward. The standard asks for the failing field to be named and the error described in text at Level A, with no exception, and for a suggested correction at Level AA wherever one is known. It is that second duty which lifts when a suggestion would give away something that has to stay secret or would defeat the point of the exercise, and that is why a login form still has to say something went wrong and does not have to say which half.
Who notices first
- People with cognitive and learning disabilities, for whom an unexplained error ends the task rather than delaying it.
- People using a screen magnifier, who see a small part of the screen at a time and will never see a red border that landed outside it.
- People with color blindness, who get nothing at all from an error marked in red and nothing else.
- Screen reader users, who need the error named in text, because a red border says nothing they can read.
- Anybody filling in a long form. Lose the submission and you start again from the top.
The common misreading
“Understandable is subjective, so it cannot really be tested.”
The criteria are as testable as any others. Does the page declare its language? Does the error name the field? Does the help link fall in the same place in the reading order across pages that share a purpose? None of that is a matter of taste. What is true is that no tool will answer any of it for you. A person has to open the page and try.
The other three principles
- PerceivableCan everyone see, hear, or read your content?
- OperableCan everyone navigate and act, keyboard included?
- RobustDoes it hold up in screen readers and assistive tech?
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