Accessibility fundamentals: perceivable, operable, understandable, robust

What it is

WCAG 2.2 groups success criteria under four principles: content must be perceivable (text alternatives, adaptable structure, distinguishable presentation), operable (keyboard accessible, enough time, navigable), understandable (readable, predictable, input assistance) and robust (compatible with assistive technologies). Criteria have levels A, AA and AAA; AA is the common target.

Why it matters

Accessibility failures exclude people and usually indicate structural problems that also hurt search engines and automated readers.

How to apply

• Text alternatives for images; none needed for purely decorative ones (alt="").
• Contrast of at least 4.5:1 for normal text and 3:1 for large text and interface components.
• Everything reachable and operable by keyboard with a visible focus indicator; no keyboard traps.
• Labels associated with form controls; error messages that name the field and the fix.
• Skip link to the main content; consistent navigation across pages; language of the page and of parts declared.
• Do not rely on colour alone to convey meaning (underline links in running text).

Pitfalls

Automated checkers find perhaps a third of the issues; keyboard-only and screen-reader walkthroughs find the rest. title attributes are not accessible names. Removing focus outlines for aesthetics fails criterion 2.4.7.