Overview
Every WCAG success criterion falls under one of four principles, known by the acronym POUR: Perceivable, Operable, Understandable, and Robust. You do not need to memorize success criterion numbers to use this. Understanding the four principles is enough to catch most real issues on an ecommerce site just by asking the right question about each one.
Perceivable
Can a user perceive the content, through sight, sound, or touch, in some form? This covers alt text for images, captions for video, and enough color contrast that text is readable. A product photo with no alt text fails here, since a screen reader user cannot perceive what the image shows at all.
Operable
Can a user operate every control, regardless of input method? This covers keyboard navigation, enough time to complete actions like checkout, and avoiding content that flashes in a way that can trigger seizures. A cart quantity stepper that only responds to a mouse click fails here, since a keyboard-only user cannot operate it.
Understandable
Can a user understand the content and how to use the interface? This covers clear labels, predictable navigation, and helpful error messages. A checkout form that rejects a card number with no explanation of what was wrong fails here, since the user cannot understand what to fix.
Robust
Will the content keep working across different browsers, devices, and assistive technologies, including ones that do not exist yet? This mostly comes down to using correct, standard HTML and ARIA so screen readers and other tools can interpret the page reliably. A custom dropdown built with generic elements and no ARIA role often fails here, since assistive technology cannot tell it is a dropdown at all.
Why this framework matters for your store
Every WCAG 2.1 AA success criterion, the exact standard courts and audits reference, sits under one of these four. When an audit finding cites a criterion number, checking which principle it falls under tells you what kind of problem it actually is. A Perceivable issue usually needs a content fix. An Operable issue usually needs a code fix to how something responds to input.
Example
A store’s audit report cited failures under all four principles on a single product page: missing alt text (Perceivable), a size-selector only usable by mouse (Operable), a shipping estimate with no label explaining what it referred to (Understandable), and a custom accordion with no ARIA attributes (Robust). Grouping the fixes by principle let the developer batch similar work instead of fixing issues in a random order.
Common mistake
Assuming visual polish means the page is accessible. A page can look clean and organized to a sighted user and still fail Operable or Robust entirely, since those two principles are largely invisible unless you specifically test with a keyboard or a screen reader. This is exactly the gap manual testing exists to catch.