The W3C's Web Content Accessibility Guidelines (WCAG) is the internationally recognized standard for creating accessible Web-based content.


The WCAG guidelines take a layered approach to accessibility, starting with four high-level principles that all content creators should strive to achieve. These principles are defined as follows:

  1. Perceivable — Information and user interface components must be presentable to users in ways they can perceive.
  2. Operable — User interface components and navigation must be operable.
  3. Understandable — Information and the operation of user interface must be understandable.
  4. Robust — Content must be robust enough that it can be interpreted reliably by a wide variety of user agents, including assistive technologies.

At the next level, each of these principles is broken down into a set of guidelines for achieving compliance.

1. Perceivable
  1. 1.1 Text Alternatives
  2. 1.2 Time-based Media.
  3. 1.3 Adaptable
  4. 1.4 Distinguishable
2. Operable
  1. 2.1 Keyboard Accessible
  2. 2.2 Enough Time
  3. 2.3 Seizures and Physical Reactions
  4. 2.4 Navigable
  5. 2.5 Input Modalities
3. Understandable
  1. 3.1 Readable
  2. 3.2 Predictable
  3. 3.3 Input Assistance
4. Robust
  1. 4.1 Compatible

A further level below the guidelines are the success criteria, which is the level where WCAG moves from a general outline for accessible content to how to practically achieve it. These criteria are quite extensive, so are not reproduced here. For more information, refer to the WCAG guidelines.

After these three levels of progressive refinement of what it takes to create accessible content, you reach the sufficient and advisory techniques for testing compliance to the success criteria. These criteria provide the format-specific information you need to verify your content.

This knowledge base focuses on the techniques for publishing, in particular as they relate to meeting the EPUB Accessibility specification, but understanding the higher-level concepts is also necessary to truly understand what makes content accessible. You will find links to the relevant techniques throughout this site, while the introductions to each section endeavour to explain why making these structures accessible is necessary (i.e., how they fit into the higher-level guidelines).


WCAG defines three levels of conformance (from 'A' to 'AAA') to indicate the breadth of support a site provides.

Level A is the minimal accessibility baseline that documents must meet, but at this level they will likely remain problematic for their target audiences. Level AA is commonly considered the actual baseline that documents must meet in order to be considered accessible.

Note that in order to meet Level AA, all of the requirements of Level A must be met. Each higher level of conformance builds on the levels below it.

Level AAA is widely regarded as aspirational, as it is typically not possible for documents to meet all of the requirements. Authors should not ignore the success criteria at this level, but may defer requirements that are too complicated to achieve.

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