Foundations

The 4 POUR Principles: Perceivable, Operable, Understandable, Robust

WCAG organizes all of its success criteria under four principles, known by the acronym POUR. Every single requirement in the spec — all 55 success criteria at A and AA — falls under exactly one of these four. Understanding POUR is the fastest way to build intuition for what WCAG actually cares about, instead of memorizing a checklist.

Perceivable

Users must be able to perceive the information being presented — it can't be invisible to all of their senses at once.

Example violation: an <img> tag with no alt attribute. A sighted user sees the picture; a screen reader user gets nothing — the information simply isn't perceivable to them. See Missing Alt Text for the fix.

Operable

Users must be able to operate the interface — every interactive control has to be usable, and users need enough time and control to interact with it safely.

Example violation: a dropdown menu that only opens onmouseover, with no keyboard equivalent. A mouse user operates it fine; a keyboard-only user (including many people with motor disabilities, and every screen reader user navigating by Tab) can't open it at all.

Understandable

Both the content and the operation of the interface must be understandable — users shouldn't be confused about what something says or what will happen when they interact with it.

Example violation: a form that shows a red asterisk next to a field but never labels what the asterisk means, or a page whose language changes mid-paragraph without being marked up — a screen reader will keep reading in the wrong language's pronunciation rules.

Robust

Content must be robust enough to be reliably interpreted by a wide range of tools, including current and future assistive technologies — not just today's dominant browser.

Example violation: custom-built interactive widgets (a "div that acts like a button") with no ARIA role, name, or state. Visually it works fine; to a screen reader's accessibility tree, it's just an anonymous, non-interactive <div>.

Why this framework matters

When you're staring at a scan result trying to decide how urgent a violation actually is, POUR is a useful gut-check: a Perceivable or Operable failure usually blocks access outright (the user genuinely cannot get the information or complete the task), while some Understandable issues are closer to friction than a hard block. It's not a formal severity scale — AllyProof's own severity ratings (critical/serious/moderate/minor) do that job — but it's the mental model WCAG itself is built on, and it's worth having before you dig into the individual success criteria.

Common questions

What does POUR stand for in accessibility?

Perceivable, Operable, Understandable, and Robust — the four top-level principles of WCAG. Every success criterion belongs to one of them.

What does each POUR principle mean?

Perceivable: users can sense the content, such as alt text and captions. Operable: they can use it with any input, such as a keyboard. Understandable: content and controls are clear. Robust: it works with current and future assistive technology.

Why do the POUR principles matter?

They are the mental model for WCAG. Grouping dozens of criteria into four goals makes it far easier to understand what accessibility is trying to achieve and where a given failure fits.