The four main characteristics of accessible design are perceivable, operable, understandable, and robust. This means that users must be able to perceive the information being presented (it can't be invisible to all of their senses), operate the interface (the interface cannot require interaction that a user cannot perform), understand the information and the operation of the user interface (the content or operation cannot be beyond their understanding), and robust enough to be interpreted reliably by a wide variety of user agents, including assistive technologies.