What are the four WCAG principles, and what is each one a claim about?
answer
- Four buckets sit above the numbered criteria
- One is about reaching the senses
- One is about driving and completing tasks
- The fourth concerns interpretation by other software
basics
~10 sWCAG organises every requirement under four principles: perceivable, operable, understandable, robust. Content must reach the user's senses, be usable without a mouse or precise pointing, be comprehensible, and be exposed reliably to assistive technologies.
solid answer
~50 sWCAG is a tree, not a checklist: four **principles**, thirteen guidelines beneath them, and numbered **success criteria** beneath those. **Perceivable** claims the information can reach a user through a sense they have. **Operable** claims every function can be reached, driven and completed — from a keyboard interface, with enough time, without flashing that triggers seizures. **Understandable** claims the content reads clearly, the interface behaves predictably, and errors are identified and explained. **Robust** claims the content is exposed so a variety of user agents, including assistive technologies, can interpret it. Only the numbered criteria are testable; the principles are the argument structure. That is why they earn their keep in review: you name the criterion to show something is a defect rather than a matter of taste, and you name the principle to say whose experience it breaks.
go deeper
Be ready to name the four principles and give one concrete failure under each. This is a screening question, so fluency matters more than depth — an unhesitating answer with examples reads far better than a hedged recitation.
Explain the three layers — principles, guidelines, numbered criteria — and which of them is actually testable. Be able to read a criterion number and say which principle it defends, and why that structure exists at all.
Show that you use the structure in review: name the criterion first, then the principle, to move a disagreement off taste. Also know where it runs out — a real barrier with no matching criterion is still a defect worth raising.
Own the framing across teams. The principles are how you explain accessibility work to people who will never read a criterion, and how you decide which classes of failure block a release rather than joining a backlog.
## Three layers, and only one of them is testable WCAG is a tree, and most confusion about it comes from standing on the wrong layer. - **Principles** sit at the top. There are four, and they are not requirements you can pass or fail. They state what accessibility is for: information must reach the user, the interface must be usable, the content must make sense, and the whole thing must survive being interpreted by software the author did not write. - **Guidelines** sit under the principles — thirteen of them across the four. They are goals, not tests: provide text alternatives, make functionality available from a keyboard interface, help users avoid and correct mistakes. You cannot audit a guideline either. - **Success criteria** sit under the guidelines. Each is a numbered, titled, testable statement carrying a conformance level. These are the only things a conformance claim is made of. The numbering encodes the tree. A criterion numbered 2.4.5 is the fifth criterion of the fourth guideline under the second principle, so a bare number already tells you which principle it defends. ## What each principle is a claim about | Principle | The claim it makes | What a failure looks like | |---|---|---| | **Perceivable** | Information and interface parts can be presented in a form the user can sense | Meaning carried only by an image, only by sound, or only by shape or position | | **Operable** | Every function can be reached, driven and completed | A control that needs a precise pointer gesture; a time limit that cannot be extended | | **Understandable** | Content reads clearly and the interface behaves predictably | An error the user cannot identify; a control that acts before it is confirmed | | **Robust** | Content is exposed so a variety of user agents, including assistive technologies, can interpret it | A custom control whose purpose and current state exist only in the visual layer | Two clarifications people usually need: - **Perceivable is not only about blindness.** It covers captions and transcripts for time-based media, structure that adapts, and content that does not depend on a single sensory characteristic — 1.3.3 Sensory Characteristics is the criterion that bans 'press the round control on the right' as the only instruction. - **Robust is the principle that decays.** It governs the contract between your content and software you do not control, and it is the smallest principle: one guideline, and one of its criteria, 4.1.1 Parsing, was marked obsolete in WCAG 2.2 as platforms improved. Operable is where most everyday interview material lives, because 2.1.1 Keyboard and the timing criteria such as 2.2.1 Timing Adjustable cover the failures that stop a task outright. Understandable is where the error and help criteria sit, including 3.3.1 Error Identification. ## Using the structure to argue defect, not preference The commonest workplace failure is not ignorance of the criteria; it is losing the argument that an issue is a bug at all. The three layers give you three moves, in this order: 1. **Name the criterion.** 'This fails 2.1.1 Keyboard' is a claim about a published, testable statement that anyone can re-check. 'This feels inaccessible' is a claim about you. 2. **Name the principle.** The criterion says what is broken; the principle says whose experience it breaks and why the breakage is categorical rather than cosmetic. A user who cannot operate a control has not had a worse experience — they have had no experience. 3. **Name the level.** If the team has already committed to a conformance target, a level A or AA failure is out of policy, which converts a value judgement into a check against a decision the organisation has already made. That sequence moves accessibility out of the register of taste — 'I would have designed it differently' — and into the register of correctness: this criterion is not satisfied on this screen. ## Where the structure stops helping - You cannot fail a principle. Nobody is ever cited for 'not perceivable'; findings attach to criteria. - Passing every criterion at a level does not make a product good. Conformance is a floor, and content can satisfy the criteria and still be tiresome to use. - Some genuine barriers have no criterion. When that happens, raise them as usability defects with evidence of who they block, rather than stretching a criterion to cover them. A stretched citation is the fastest way to lose credibility on the next real finding. A last note on vocabulary, because it costs people interviews. 'Principle' here means one of the four, not a team design principle. 'Level' means a conformance level, not a heading level or a seniority level. 'Criterion' means a numbered success criterion, not an acceptance criterion on a story. Say which you mean in a mixed room and you will save more time than any technical detail buys you.
- Which layer of WCAG do you actually cite in a bug report, and why?The success criterion — number, title and level. Principles and guidelines are not testable, so a finding written against a principle cannot be verified or closed. Naming the criterion gives the fix a definition of done and lets anyone re-test it. The principle is what you say next, to explain who the failure affects and why it is not cosmetic.
- A colleague says the barrier you found is not covered by any success criterion. What do you do?Check first — many barriers map to a criterion people simply do not know. If none genuinely applies, do not stretch one; a bad citation costs credibility on the next real finding. Raise it as a usability defect with evidence of who it blocks and price it on impact. Conformance is a floor, not the definition of usable.
- How does a criterion's number tell you which principle it belongs to?The first digit is the principle, the second the guideline, the third the criterion's position under it. A criterion numbered 3.3.1 therefore defends the understandable principle through its third guideline. That makes a bare number readable in a triage list without opening the standard, which is why findings are worth recording by number.
Think of it as a building code: the principles are why the code exists, and the numbered criteria are the clauses an inspector actually signs off.
saying these in an interview costs you the question
- Recites the four principles but gives no example of any
- Treats a principle as something a page can pass or fail
- Thinks accessibility means only blind users and screen readers
- Calls an inoperable control a design preference rather than a defect
- Believes robust simply means code that a validator accepts