The “Understanding WCAG 2.0” webpage, provided by the W3C (World Wide Web Consortium), explains that for a webpage to be accessible, it must meet four criteria. It needs to be:
Perceivable - Information and user interface components must be presentable to users in ways they can perceive.
This means that users must be able to perceive the information being presented (it can't be invisible to all of their senses)
Operable - User interface components and navigation must be operable.
This means that users must be able to operate the interface (the interface cannot require interaction that a user cannot perform)
Understandable - Information and the operation of user interface must be understandable.
This means that users must be able to understand the information as well as the operation of the user interface (the content or operation cannot be beyond their understanding)
This means that users must be able to access the content as technologies advance (as technologies and user agents evolve, the content should remain accessible)
(“Understanding WCAG 2.0”, W3C)
It is important to note that although the WCAG guidelines are specifically focused on web-based technology, these four principles can be used to guide the creation of any type of document or media in order to ensure that it is available and usable to the largest possible audience. The techniques used to make a document accessible will be similar in all of the document types we will discuss below.