The principles of accessibility that the World Wide Web Consortium process (W3C) created for web users:

POUR is an abbreviation for four significant level rules that describe functional accessibility. Perceivable, Operable, Understandable, and Robust. By applying those principles, websites and any kind of technology can be made accessible.

P for Perceivable

“Information and user interface components must be presentable to users in ways they can perceive.”

O for Operable

“User interface components and navigation must be operable.”

U for Understandable

“Information and the operation of user interface must be understandable.”

R for Robust

“Content must be robust enough that it can be interpreted by a wide variety of user agents, including assistive technologies.”