Can users actually understand your interface? That's the critical question.
Information and user interface operation must be understandable. Readable text using plain language—8th-grade reading level recommended. Declared language enabling pronunciation. Unusual words defined, abbreviations expanded. Predictable behavior maintaining consistent navigation and identification. No context changes on focus or input without warning. Error identification in text descriptions, not color alone. Input instructions provided before submission. Error suggestions offered when known. Error prevention for legal, financial, or data submissions requiring confirmation or review.
W3C WCAG 2.2 establishes Understandable as the third POUR principle. The goal? Users with cognitive disabilities, learning disabilities, or language differences must comprehend content and interface operation.
The research validates it. Sites meeting Understandable principle achieve 50-70% better comprehension for users with cognitive disabilities, 40-60% fewer completion errors through clear instructions and error handling, and 30-50% reduced support costs through predictable behavior and helpful guidance. Understandability proves essential for universal comprehension—regardless of cognitive capabilities or language proficiency.
W3C WCAG 2.2 (2023) established Understandable as third fundamental POUR principle (Perceivable, Operable, Understandable, Robust) addressing content comprehension and interface predictability. Critical insight: users who perceive and operate content cannot accomplish goals without understanding information and predicting interface behavior—perfectly accessible keyboard navigation proves useless with incomprehensible instructions, excellent screen reader support failing with unclear error messages, complete perceivability and operability negated by unpredictable behaviors creating confusion and task abandonment. Understandable principle encompasses three guidelines ensuring comprehension across cognitive abilities serving learning disabilities (dyslexia, dyscalculia, ADHD, autism spectrum), acquired cognitive impairments (traumatic brain injury, dementia, stroke), temporary conditions (stress, distraction, fatigue), second language speakers requiring clear communication.
Guideline 3.1: Readable requires text content be readable and understandable. SC 3.1.1 (Level A): Language of Page—HTML lang attribute declaring primary content language enabling assistive technologies correct pronunciation, voice synthesis, braille translation. SC 3.1.2 (Level AA): Language of Parts—lang attribute for passages differing from primary language enabling proper pronunciation of multilingual content. SC 3.1.3 (Level AAA): Unusual Words—mechanisms identifying specific definitions of words used in unusual/restricted ways including jargon, idioms, slang. SC 3.1.4 (Level AAA): Abbreviations—expanded form or definition for abbreviations. SC 3.1.5 (Level AAA): Reading Level—when text requires reading ability beyond lower secondary education (8th-9th grade, age 13-15), supplemental content or alternative versions provided. Research demonstrating plain language improving comprehension 30-50% for users with cognitive disabilities, 25-40% for general population, reducing completion time 40-60% through clearer instructions, achieving 50-70% fewer errors versus complex language creating confusion.
Guideline 3.2: Predictable requires web pages operate in predictable ways. SC 3.2.1 (Level A): On Focus—receiving keyboard focus does not initiate context change (new window opening, form submission, focus movement to different element) unless user warned. SC 3.2.2 (Level A): On Input—changing setting of user interface component (dropdown selection, checkbox toggle, radio button selection) does not automatically cause context change unless user warned or change clearly indicated before interaction. SC 3.2.3 (Level AA): Consistent Navigation—navigation mechanisms repeated across pages appear in same relative order unless user-initiated change. SC 3.2.4 (Level AA): Consistent Identification—components with same functionality identified consistently across pages (search labeled "Search" everywhere versus "Search," "Find," "Locate" variations creating confusion). Research validating consistency improving task completion 30-50% through reduced cognitive load and established expectations, reducing errors 40-60% through predictable behaviors matching mental models, improving efficiency 25-40% through eliminated relearning.
Guideline 3.3: Input Assistance helps users avoid and correct mistakes. SC 3.3.1 (Level A): Error Identification—errors automatically detected and described to user in text (not solely through color), identifying specific fields with errors. SC 3.3.2 (Level A): Labels or Instructions—labels or instructions provided when content requires user input including required fields, format requirements, character limits. SC 3.3.3 (Level AA): Error Suggestion—if input error automatically detected and correction suggestions known, suggestions provided to user unless security/purpose compromised (password changes, CAPTCHA where suggestions defeat purpose). SC 3.3.4 (Level AA): Error Prevention (Legal, Financial, Data)—submissions causing legal commitments, financial transactions, user data modifications are reversible, checked before submission, or confirmed before final submission. WCAG 2.2 addition: SC 3.3.7 (Level A): Redundant Entry—information previously entered within same process not required again unless re-entry essential or security necessary. Research demonstrating clear error messages reducing support contacts 30-50%, error suggestions improving first-attempt success 40-60%, error prevention reducing costly mistakes 50-70% in legal/financial contexts.
Contemporary accessibility research emphasizing cognitive accessibility addressing neurodiversity—autism spectrum requiring literal language avoiding idioms, ADHD requiring focus management preventing distractions, dyslexia requiring readable typography with adequate spacing, intellectual disabilities requiring simple step-by-step instructions. Cognitive accessibility benefits extending universally—clear language helping second-language speakers, predictable behavior reducing cognitive load during stress/distraction, helpful error messages benefiting users in unfamiliar domains. Studies showing cognitive accessibility improvements benefiting 100% users not just those with diagnosed disabilities through reduced complexity, clearer communication, improved usability.