Accessible design is based on four principles: perceivable, operable, understandable, and robust. These principles are defined by the Web Content Accessibility Guidelines (WCAG), a set of standards and best practices for web accessibility. Perceivable means that the information and interface components are presented in ways that users can perceive, such as using text alternatives, captions, and contrast. Operable means that the interface components are easy to use and navigate, such as using keyboard access, focus indicators, and skip links. Understandable means that the information and interface components are clear and consistent, such as using simple language, labels, and feedback. Robust means that the content and interface components are compatible with different devices, browsers, and assistive technologies, such as using valid code, markup, and ARIA attributes.