POUR
Also: Perceivable Operable Understandable Robust
The four principles WCAG is built on: **Perceivable, Operable, Understandable, Robust.** Every success criterion maps to one of these.
POUR is the conceptual frame WCAG is built on. Every success criterion in WCAG 2.x maps to exactly one of the four principles. Memorising POUR is the shortest path to understanding why a given rule exists.
The four principles
Perceivable. Information and interface components must be presentable to users in ways they can perceive. In practice: text alternatives for non-text content (images, videos), captions and transcripts for audio, sufficient colour contrast, content that adapts to screen-reader and zoomed-layout users. If a sighted user can see something a blind user cannot, that is a perceivability gap.
Operable. User interface components and navigation must be operable. The big rules under this principle: full keyboard access (no mouse-dependent features), enough time to read and complete tasks, no seizure-inducing content (flashing > 3 times per second is the bright line), clear navigation, and predictable focus order. WCAG 2.2’s new focus-visibility and drag-alternative criteria are operability rules.
Understandable. Information and the operation of the interface must be understandable. This covers things like predictable navigation patterns, consistent labelling across pages, clear instructions, and helpful error recovery. The cognitive-accessibility working group’s recommendations slot under this principle, even when WCAG itself stops short of mandating them.
Robust. Content must be robust enough to be interpreted reliably by a wide variety of user agents, including assistive technologies. In practice this is the smallest principle, with mostly one rule — 4.1.2 Name, Role, Value — but that one rule covers an enormous surface: it requires that every custom UI control expose what it is, what it does, and what state it’s in, in a way that assistive tech can read. 4.1.2 is why ARIA exists.
How to use POUR operationally
When you find an accessibility issue, ask which principle it violates:
- A missing alt attribute is Perceivable.
- A keyboard trap is Operable.
- A confusing error message is Understandable.
- A custom dropdown that doesn’t announce as a combobox is Robust.
That mapping helps prioritise: Perceivable and Operable failures usually block users entirely; Understandable and Robust failures often degrade the experience without blocking it. Most teams focus their first audit pass on P and O for that reason.
POUR also helps spot gaps. A team that obsessively fixes Perceivable issues (alt text, contrast) but ignores Operable ones (focus order, keyboard traps) has a recognisable, common failure pattern. POUR makes that lopsidedness visible.