Perceivable, operable, understandable, robust

The four major principles of WCAG 2.0, a set of web accessibility guidelines published by the World Wide Web Consortium (W3C). Information and user interface components must be presentable to users in ways they can perceive; user interface components and navigation must be operable; information and the operation of user interface must be understandable; and content must be robust enough that it can be interpreted reliably by a wide variety of user agents, including assistive technologies. ← Wikipedia