All 87 WCAG 2.2 success criteria are organized under four top-level principles, known by the acronym POUR: Perceivable, Operable, Understandable, and Robust. Every accessibility violation maps back to one of these principles. Understanding them helps you reason about accessibility beyond checklist compliance.
1. Perceivable
Information and UI components must be presentable to users in ways they can perceive. This means content cannot be invisible to all of a user's senses. Key criteria under Perceivable include alt text for images (1.1.1), captions for video (1.2.2), sufficient color contrast (1.4.3), and content that doesn't disappear when text is resized (1.4.4). If a user cannot see and the content has no text alternative, it fails Perceivable.
2. Operable
UI components and navigation must be operable by everyone. This addresses users who cannot use a mouse, have motor tremors, or interact through voice control or switch devices. Key criteria include full keyboard accessibility (2.1.1), no keyboard traps (2.1.2), sufficient time for timed interactions (2.2.1), and the 24px minimum target size added in 2.2 (2.5.8). If a feature only works with a precise mouse click, it fails Operable.
3. Understandable
Content and UI operations must be understandable. This covers readability, predictability, and input assistance. Key criteria include declaring the page language (3.1.1), preventing unexpected context changes on focus (3.2.1), clear error identification (3.3.1), and labels for form inputs (3.3.2). A form that fails silently when submitted incorrectly, with no error message, fails Understandable.
4. Robust
Content must be robust enough to be interpreted reliably by a wide variety of user agents, including assistive technologies. The primary criterion is 4.1.2: Name, Role, Value — every UI component must expose its name, role, and state programmatically. Custom interactive components built without ARIA are the most common source of Robust failures.
Tip
When reviewing a scan report, group violations by POUR principle to understand where your systemic weaknesses are. A pattern of Perceivable failures usually means missing alt text or contrast issues. Operable failures often indicate custom components without keyboard support.