What are the four fundamental principles of WCAG, and what does each one mean?

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WCAG, Web Content Accessibility Guidelines, are the international web accessibility standards developed by the W3C. They are organized around four fundamental principles, often memorized with the acronym POUR.

The first principle is Perceivable. Content must be presentable in ways users can perceive. This means providing text alternatives for non-text content, such as alt attributes for images. Provide captions and transcripts for audio and video content. Ensure content is adaptable and can be presented in different ways without losing information. Guarantee sufficient contrast between text and background.

The second principle is Operable. The interface and navigation must be operable. This means making all functionality accessible via keyboard. Give users enough time to read and interact. Do not design content that could cause seizures, like rapid flashes. Help users navigate and find content.

The third principle is Understandable. Content and interface must be understandable. This means making text readable and comprehensible. Make pages function predictably. Help users avoid and correct errors, especially in forms.

The fourth principle is Robust. Content must be robust enough to be interpreted by a wide variety of user agents, including assistive technologies. This means using valid HTML and following standards.