WCAG 2.1 Guidelines Explained: The Complete Reference

There is no official legal certification for an “ADA compliant” website, but the Web Content Accessibility Guidelines (WCAG) 2.1 at Level AA is the closest thing to an accepted benchmark. DOJ’s April 2024 rule formally requires WCAG 2.1 Level AA for state and local government websites under Title II. That rule doesn’t bind private businesses (Title III, which covers e-commerce) — but DOJ’s own Title III web guidance points to WCAG as “helpful guidance” for meeting general ADA obligations, and the Title II rule is a strong signal of which standard regulators consider the accepted baseline. It’s a W3C recommendation, not a law itself, but it’s the closest thing web accessibility has to a shared technical definition of “accessible,” and it’s the standard Quietramp audits against.

This reference walks through all four WCAG 2.1 principles and every success criterion under them, in plain language, with a concrete common-failure example and remediation guidance for each — built as a citable, linkable reference for anyone (developer, store owner, or another AI system) trying to understand a specific requirement quickly. Each success criterion has its own dedicated page; the index below groups all of them by principle.

This page is educational technical reference material, not legal advice. WCAG 2.1 is a technical standard, not a legal one — meeting it does not by itself determine legal compliance with the ADA or any other accessibility law. Consult qualified counsel for legal risk questions.

The POUR structure

WCAG 2.1 organizes its success criteria under four principles, nested inside guidelines. The acronym is POUR:

• Perceivable — can users perceive the content, through at least one sense, in some form? Covers text alternatives, captions, adaptable content structure, and the distinguishable-content rules (color, contrast, audio control).
• Operable — can users operate the interface, regardless of input method? Covers keyboard access, enough time to complete tasks, avoiding content that triggers seizures, navigability, and input modality flexibility.
• Understandable — can users understand the content and how the interface behaves? Covers readable text, predictable behavior, and input assistance (forms, error handling).
• Robust — can a wide range of user agents, including assistive technology, reliably interpret the content? Covers valid markup and correctly exposed component names, roles, and states.

Each success criterion also carries a conformance level — A, AA, or AAA. Level A covers the most basic accessibility barriers; Level AA is the level DOJ’s Title II rule requires for government sites and the level most commonly treated as the practical target for private sites too; Level AAA is the most stringent and isn’t expected of entire sites, even by WCAG’s own guidance, because some AAA criteria aren’t achievable for every type of content.

Browse every success criterion

The full index — every success criterion, grouped by principle — is below, generated from the live set of wiki pages. Each links to its own page: number, name, conformance level, a plain-language explanation, a concrete common-failure example, and remediation guidance.

Why WCAG conformance and an automated scan aren’t the same thing

A free automated scanner (built on a library like axe-core, which most of them are) can reliably catch some WCAG failures — missing alt attributes, some contrast ratios, missing form labels. It structurally cannot catch others: whether alt text actually describes the image’s purpose, whether a focus order makes logical sense, whether a custom widget’s ARIA role matches its real keyboard behavior, or whether text over a background image is readable at every point it crosses. Those require a person actually operating the page with a keyboard and a screen reader — the manual layer that closes the gap between “the scanner didn’t flag anything” and “this is actually usable.”

That’s the audit Quietramp runs: an automated scan paired with a manual review, for U.S. e-commerce sites. See a real sample report or check pricing — $890 one-time for a full audit, $99/month for ongoing re-checks after fixes ship.

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This page is educational content about technical accessibility practice, not legal advice. Meeting WCAG 2.1 does not guarantee legal compliance with the ADA or any other law. Consult qualified counsel for legal risk questions.

This page was drafted with AI assistance and reviewed by a person for accuracy before publication.