Making content perceivable means making the output
available to the user's senses, namely sight, sound, and touch
(in the case of people who use Braille output devices). We won't worry about
tasting or smelling web pages!
Making content operable means making the input mechanisms
robust enough to accept a wide range of devices and methods, including keyboard,
mouse, touch, gestures, single-switch devices, and so on.
Making content understandable means making the message and the interface
easy to use and comprehend.
Making content robust means ensuring it works across a wide range of
devices, with both forward and backward compatibility.