Perceivable means information can’t be invisible to any one sense, so images need text alternatives and video needs captions. Operable means all functionality works from a keyboard, with enough time to read and use content, and without triggering seizures. Understandable means text is readable and interfaces behave predictably, including clear error messages on forms.
Robust means content works reliably across current and future browsers and assistive technologies, largely through valid, semantic code. Every WCAG success criterion falls under one of these four principles, which makes POUR a useful mental model even for people who haven’t memorized the full guidelines.