Textbook of Usability

Also known as: Web Content Accessibility Guidelines

The Web Content Accessibility Guidelines (WCAG) are the international standard for web accessibility, published by the World Wide Web Consortium (W3C). The current versions (WCAG 2.1 and 2.2) are organised around four principles, collectively known as POUR:

• Perceivable — information must be presentable in ways users can perceive (text alternatives for images, captions for video, sufficient contrast)
• Operable — interface components must be operable by all users (keyboard accessibility, sufficient time, no seizure-inducing content)
• Understandable — information and operation must be understandable (readable text, predictable operation, input assistance)
• Robust — content must work with a wide variety of user agents, including assistive technologies (valid semantic HTML, ARIA attributes)

WCAG organises requirements into three conformance levels:

• Level A — minimum (required for basic accessibility)
• Level AA — standard (most commonly required by law and policy)
• Level AAA — enhanced (additional support for users with more significant disabilities)

Specific success criteria include minimum contrast ratios (4.5:1 for normal text, 3:1 for large text), text alternatives for non-text content, keyboard operability for all functionality, and sufficient timing for interaction.

WCAG is referenced in legal frameworks worldwide: the Americans with Disabilities Act (US), Equality Act 2010 (UK), European Accessibility Act (EU), and many others. WCAG 2.1 Level AA is the most commonly cited compliance target.

Accessibility is not a concession to a minority — it is fundamental to usability and often improves the experience for all users.