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Understanding WCAG 2.0

Rather than issuing a simple checklist of "do’s and don'ts," WCAG 2 instead establishes a series of Success Criteria to address various online content barriers. The Success Criteria are written as testable statements that are not technology-specific. This approach ensures that content authors are not told that they cannot do a particular thing, but instead offers means and suggestions for ensuring that whatever is created can be made accessible.

Understanding the Four Principles of Accessibility

The guidelines and Success Criteria are organized around the following four principles, which lay the foundation necessary for anyone to access and use web content. Anyone who wants to use the Web must have content that is:

  • 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).
  • Robust - "Content must be robust enough that it can be interpreted reliably by a wide variety of user agents, including assistive technologies." 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).

If any of these are not true, users with disabilities will not be able to use the Web.

Under each of the principles are guidelines and Success Criteria that help to address these principles for people with disabilities. There are many general usability guidelines that make content more usable for everyone, including those with disabilities. However, in WCAG 2.0, we only include those guidelines that address problems particular to people with disabilities. This includes issues that block access or interfere with access to the Web more severely for people with disabilities.

The W3C Success Criteria is an online resource that has been assembled by a group of experts in the field of online accessibility. Due to its online nature, it remains a living resource that is modified and updated to address the rapid advances in Internet technologies, ensuring that new technologies are addressed, but also providing the most current best practices and examples, including future solutions that might not today be available. 

A quick reference guide can be found at:

After looking at the criteria briefly, you will notice that rather than admonishing the content creator to do something in a specific way, the criteria offer multiple suggestions on how to ensure that that information can be conveyed accessibly.

Keep in mind that techniques are informative, not normative, which means that other techniques may be used, as long as the success criterion is met. This methodology seeks to instruct and teach basic principles rather than restrict and stifle creativity and freedom.

Conformance Requirements

In order for a web page to conform to WCAG 2.0, all of the following conformance requirements must be satisfied:

  1. Conformance Level: One of the following levels of conformance is met in full.
    • Level A: For Level A conformance (the minimum level of conformance), the web page satisfies all the Level A Success Criteria, or a conforming alternate version is provided.
    • Level AA: For Level AA conformance, the web page satisfies all the Level A and Level AA Success Criteria, or a Level AA conforming alternate version is provided.
    • Level AAA: For Level AAA conformance, the web page satisfies all the Level A, Level AA and Level AAA Success Criteria, or a Level AAA conforming alternate version is provided.

Note 1: Although conformance can only be achieved at the stated levels, authors are encouraged to report in their claim any progress toward meeting success criteria from all levels beyond the achieved level of conformance.

Note 2: It is not recommended that Level AAA conformance be required as a general policy for entire sites because it is not possible to satisfy all Level AAA Success Criteria for some content.

Stanford University makes an effort to assure that its websites and services are accessible to all. When sites are assessed, they are reviewed against the WCAG 2.0 AA criteria.

Last modified: 
July 11, 2014