The four principles of accessibility are Perceivable, Usable, Understandable, and Robust.
Perceivable means that the data and components of the user interface must be presentable to users in a way that they can perceive. Usable means that the user interface must be operable using a keyboard or other device. Understandable means that the information and operation of the user interface must be understandable, and robust means that the content must be robust enough to be interpreted by a variety of users.