Introduction to Understanding WCAG 2.1

Understanding WCAG 2.1 is an essential guide to understanding and using "Web Content Accessibility Guidelines 2.1". Although the normative definition and requirements for WCAG 2.1 can all be found in the WCAG 2.1 document itself, the concepts and provisions may be new to some people. Understanding WCAG 2.1 provides a non-normative extended commentary on each guideline and each Success Criterion to help readers better understand the intent and how the guidelines and Success Criteria work together. It also provides examples of techniques or combinations of techniques that the Working Group has identified as being sufficient to meet each Success Criterion. Links are then provided to write-ups for each of the techniques.

This is not an introductory document. It is a detailed technical description of the guidelines and their Success Criteria. See Web Content Accessibility Guidelines (WCAG) Overview for an introduction to WCAG, supporting technical documents, and educational material.

Understanding WCAG 2.1 is organized by guideline. There is an Understanding Guideline X.X section for each guideline. The intent and any advisory techniques that are related to the guideline but not specifically related to any of its Success Criteria are listed there as well.

The Understanding Guidelines X.X section is then followed by a Understanding Success Criterion X.X.X section for each Success Criterion of that guideline. These sections each contain:

Links are provided from each Guideline in WCAG 2.1 directly to each Understanding Guideline X.X in this document. Similarly, there is a link from each Success Criterion in WCAG 2.1 to the Understanding Success Criterion X.X.X section in this document.

For information about individual techniques, follow the links throughout this document to the techniques of interest in the IN DEVELOPMENT: Techniques for WCAG 2.1 document.

For links to information on different disabilities and assistive technologies see Disabilities on Wikipedia.

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:

  1. 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)

  2. 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)

  3. 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)

  4. 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 by all people, including those with disabilities. However, in WCAG 2.1, 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.

Layers of Guidance

The Guidelines

Under each principle there is a list of guidelines that address the principle. There are a total of 12 guidelines. A convenient list of just the guidelines can be found in the WCAG 2.1 table of contents. One of the key objectives of the guidelines is to ensure that content is directly accessible to as many people as possible, and capable of being re-presented in different forms to match different peoples' sensory, physical and cognitive abilities.

Success Criteria

Under each guideline, there are Success Criteria that describe specifically what must be achieved in order to conform to this standard. They are similar to the "checkpoints" in WCAG 1.0. Each Success Criterion is written as a statement that will be either true or false when specific Web content is tested against it. The Success Criteria are written to be technology neutral.

All WCAG 2.1 Success Criteria are written as testable criteria for objectively determining if content satisfies the Success Criteria. While some of the testing can be automated using software evaluation programs, others require human testers for part or all of the test.

Although content may satisfy the Success Criteria, the content may not always be usable by people with a wide variety of disabilities. Professional reviews utilizing recognized qualitative heuristics are important in achieving accessibility for some audiences. In addition, usability testing is recommended. Usability testing aims to determine how well people can use the content for its intended purpose.

The content should be tested by those who understand how people with different types of disabilities use the Web. It is recommended that users with disabilities be included in test groups when performing human testing.

Each Success Criterion for a guideline has a link to the section of the How to Meet document that provides:

  • sufficient techniques for meeting the Success Criterion,

  • optional advisory techniques, and

  • descriptions of the intent of the Success Criteria, including benefits, and examples.

Sufficient Techniques, Advisory Techniques, and Failures

The next section, Understanding Techniques for WCAG Success Criteria, provides important information about the techniques.