Understandable -- WCAG Principle 3

Information and the operation of the user interface must be understandable.

What Does "Understandable" Mean?

Understandable is the third of the four top-level principles in WCAG 2.2 [1], which organise its success criteria under the POUR acronym -- Perceivable, Operable, Understandable, Robust. The principle states that information and the operation of the user interface must be understandable. Where Perceivable governs whether content reaches the user's senses and Operable governs whether the user can drive the controls, Understandable governs whether the user can comprehend what the content says and predict what the interface will do next. A page can render every pixel and accept every keystroke and still fail here if the prose is written above the reader's language level, if focusing a control silently submits a form, or if an error message reads ERR_VALIDATION_7 with no guidance on what to change.

The failure mode is concrete: a page served in English with no lang attribute forces a screen reader to guess pronunciation rules and fall back to the synthesiser's default, mangling every word; a select element wired to onchange="form.submit()" triggers navigation the moment a keyboard user arrow-keys through the options, violating 3.2.2 on context change; a primary nav whose link order differs on every page defeats the mental map a returning user has built; a checkout form that highlights invalid fields in red with no text identification strands any user who cannot see the colour cue and any user whose screen reader does not announce the border-colour change. Each is an understandability failure because the information and the interaction model are present but the reader cannot build a stable model of either.

The principle decomposes into three guidelines. 3.1 Readable governs the human-language layer -- page and passage language declarations, unusual-word glossaries, abbreviation expansions, reading-level alternatives, and pronunciation hints. 3.2 Predictable governs interaction consistency -- that focusing a control does not trigger a context change, that inputting data does not navigate the page without warning, that navigation order and component identification stay stable across a set of pages, that help is exposed in a consistent location, and that disruptive changes are initiated by the user. 3.3 Input Assistance governs error prevention and recovery -- that fields are labelled and instructed, that errors are identified in text, that suggestions are offered when the system can infer them, that legal, financial, and data-modification submissions can be reversed, checked, or confirmed, that users are not forced to re-enter information they have already supplied, that cognitive-function tests are not the sole authentication path, and that accessible authentication does not depend on puzzle-solving. Each guideline expands into individual success criteria rated A (must), AA (should for most legal targets), or AAA (aspirational).

Guidelines Under Understandable

3.1 Readable

Guideline 3.1 is the human-language guideline. 3.1.1 Language of Page (Level A) [2] requires that the default human language of each page be set programmatically, typically via <html lang="en">; the failure mode is a screen reader falling back to its synthesiser default and pronouncing French content with English phonemes, or vice versa. 3.1.2 Language of Parts (Level AA) [3] extends the same requirement to passages and phrases in a different language, via lang on the containing element. At Level AAA, 3.1.3 Unusual Words [4] requires a mechanism to identify definitions for jargon and idioms, 3.1.4 Abbreviations [5] requires expansions for abbreviations, 3.1.5 Reading Level [6] requires supplemental content or a simpler version when the prose exceeds lower-secondary education level, and 3.1.6 Pronunciation [7] requires a mechanism to disambiguate pronunciation where meaning depends on it. The guideline protects every reader whose assistive technology makes language-dependent decisions (pronunciation, hyphenation, quotation marks) and every reader whose literacy, first language, or cognitive profile places a ceiling on the reading level they can process without help.

3.2 Predictable

Guideline 3.2 governs the interaction-consistency layer -- the implicit contract a user builds with a site about what each action will do. 3.2.1 On Focus (Level A) [8] forbids a context change (new window, focus move, significant content reconfiguration, form submission) from being triggered by focus alone; the failure mode is a select element or link that submits or navigates the instant it receives focus, trapping keyboard users who cannot inspect an option without committing to it. 3.2.2 On Input (Level A) [9] extends the same prohibition to changing a control's setting -- toggling a checkbox, selecting a radio, choosing a select option -- unless the user has been warned of the behaviour before using the control. At Level AA, 3.2.3 Consistent Navigation [10] requires that repeated navigational components (primary nav, breadcrumbs, footer) appear in the same relative order on every page in a set, and 3.2.4 Consistent Identification [11] requires that components with the same functionality be labelled the same way site-wide (a search input labelled "Search" on one page and "Find" on another breaks the contract). 3.2.6 Consistent Help (Level A, added in WCAG 2.2) [12] requires that help mechanisms -- contact details, chatbots, human-assistance links, self-help links -- appear in the same relative order across pages that provide them. 3.2.5 Change on Request (Level AAA) [13] tightens the floor further, requiring that context changes be initiated only by explicit user request or be reversible. The through-line: the mental model a user builds on page one should survive to page two.

3.3 Input Assistance

Guideline 3.3 is the forms and error-recovery guideline. 3.3.1 Error Identification (Level A) [14] requires that when an input error is detected, the item in error be identified and the error described to the user in text -- not through colour or icon alone; the failure mode is a checkout form that paints invalid fields red and shows no text explanation, leaving screen-reader users and colour-blind users with no way to locate the problem. 3.3.2 Labels or Instructions (Level A) [15] requires that user-input fields carry labels or instructions that tell the user what the field expects, not a placeholder that disappears on focus. At Level AA, 3.3.3 Error Suggestion [16] requires that when the system knows how to correct an error, the suggestion be offered to the user; 3.3.4 Error Prevention (Legal, Financial, Data) [17] requires that submissions with legal, financial, or data-modification consequences be reversible, checked for errors, or confirmed before final commit. WCAG 2.2 adds two Level A criteria at the bottom of this guideline: 3.3.7 Redundant Entry [18] requires that information the user has already entered in the same session be auto-populated or selectable rather than re-entered, and 3.3.8 Accessible Authentication (Minimum) requires that no step in an authentication process depend on a cognitive function test (remembering a password, solving a puzzle, transcribing characters from an image) unless an alternative or mechanism is provided. At Level AAA, 3.3.5 Help [19] requires context-sensitive help, 3.3.6 Error Prevention (All) [20] extends 3.3.4 to every submission, and 3.3.9 Accessible Authentication (Enhanced) removes the object-recognition and personal-content exceptions allowed at the AA floor. The guideline treats the user as fallible and the interface as responsible for preventing, catching, and recovering from the mistakes that will happen.

References

  1. [1] W3C (2023). Web Content Accessibility Guidelines (WCAG) 2.2. W3C, Accessed 2026-04-07. https://www.w3.org/TR/WCAG22/ ↩
  2. [2] W3C (2023). Understanding Success Criterion 3.1.1: Language of Page. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/language-of-page.html ↩
  3. [3] W3C (2023). Understanding Success Criterion 3.1.2: Language of Parts. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/language-of-parts.html ↩
  4. [4] W3C (2023). Understanding Success Criterion 3.1.3: Unusual Words. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/unusual-words.html ↩
  5. [5] W3C (2023). Understanding Success Criterion 3.1.4: Abbreviations. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/abbreviations.html ↩
  6. [6] W3C (2023). Understanding Success Criterion 3.1.5: Reading Level. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/reading-level.html ↩
  7. [7] W3C (2023). Understanding Success Criterion 3.1.6: Pronunciation. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/pronunciation.html ↩
  8. [8] W3C (2023). Understanding Success Criterion 3.2.1: On Focus. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/on-focus.html ↩
  9. [9] W3C (2023). Understanding Success Criterion 3.2.2: On Input. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/on-input.html ↩
  10. [10] W3C (2023). Understanding Success Criterion 3.2.3: Consistent Navigation. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/consistent-navigation.html ↩
  11. [11] W3C (2023). Understanding Success Criterion 3.2.4: Consistent Identification. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/consistent-identification.html ↩
  12. [12] W3C (2023). Understanding Success Criterion 3.2.6: Consistent Help. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/consistent-help.html ↩
  13. [13] W3C (2023). Understanding Success Criterion 3.2.5: Change on Request. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/change-on-request.html ↩
  14. [14] W3C (2023). Understanding Success Criterion 3.3.1: Error Identification. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/error-identification.html ↩
  15. [15] W3C (2023). Understanding Success Criterion 3.3.2: Labels or Instructions. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/labels-or-instructions.html ↩
  16. [16] W3C (2023). Understanding Success Criterion 3.3.3: Error Suggestion. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/error-suggestion.html ↩
  17. [17] W3C (2023). Understanding Success Criterion 3.3.4: Error Prevention (Legal, Financial, Data). W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/error-prevention-legal-financial-data.html ↩
  18. [18] W3C (2023). Understanding Success Criterion 3.3.7: Redundant Entry. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/redundant-entry.html ↩
  19. [19] W3C (2023). Understanding Success Criterion 3.3.5: Help. W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/help.html ↩
  20. [20] W3C (2023). Understanding Success Criterion 3.3.6: Error Prevention (All). W3C, Accessed 2026-04-07. https://www.w3.org/WAI/WCAG22/Understanding/error-prevention-all.html ↩