The Perceivable principle states that information and user interface components must be presentable to users in ways they can perceive. This means providing text alternatives for images through alt text, supplying captions for video content, ensuring sufficient color contrast between text and backgrounds, and never relying on color alone to convey meaning. Perceivable design also extends to making content that does not break when users resize text or change spacing, since some readers need larger text or extra letter spacing to read comfortably.
The Operable principle requires that interface components and navigation must be operable by all users. Keyboard accessibility is the cornerstone of operability, since many people cannot use a mouse and rely on keys such as Tab, Shift+Tab, Enter, Space, and the arrow keys to interact with pages. Operable interfaces also avoid content that flashes more than three times per second to prevent seizure triggers, give users enough time to read content, and provide ways to pause or stop motion. Touch targets should be large enough to tap reliably, with WCAG 2.5.8 setting a 24 by 24 pixel AA minimum and 2.5.5 recommending 44 by 44 pixels at AAA.
The Understandable principle covers both information and operation: content must be readable and predictable, and interfaces must help users avoid mistakes. Plain language reduces cognitive load, headings and labels create predictable structure, and specific error messages tell users what went wrong and how to fix it rather than leaving them to guess. The Robust principle requires that content remains interpretable as technologies evolve, which means using valid markup, leveraging native HTML semantics where possible, and supplementing with ARIA only when no native element can express the right meaning.