The Four Principles of Accessibility

The Four Principles of Accessibility

All of the recommendations on accessibility best practices are derived from four base principles which help to provide the foundation of al things web accessibility. When working together, each of of the four principles and their concepts help to ensure an accessible user experience.

Right before we get into the principles and their definitions, it's good to know where they come from and who is responsible for the creation of and maintenance of this information.


The World Wide Web Consortium (W3C) is an international community that develops open standards to ensure the long-term growth of the web. These standards include things such as the HTML and CSS specifications, as well as standards for web payments, commerce, and security.

One key group formed from the W3C is the Web Accessibility Initiative (WAI). This group is responsible for creating new content for and maintenance of the Web Content Accessibility Guidelines (WCAG — also pronounced "wuh-cag").

WCAG is a worldwide recognized ISO (International Standards Organization) standard for testing and implementing accessible user experiences for people with disabilities. It's used for desktop web, mobile web and native apps, and pretty much any type of digital content.

There are country/governmental specific policies that exist to serve those regions. However, the latest version of the WCAG guidelines, WCAG 2.1, is what we'll be using as the basis of all recommendations.


The basis of each accessibility recommendation or best practice is actually derived from one of the four principles of accessibility. Each principal represents a set of guidelines which are to be followed in order to create a usable and accessible experience.

At a high-level, the principals include:

  • Perceivable — Can the user identify information and relationships of content? People must be able to perceive the information being presented – it cannot be invisible to all of their senses. Examples include: Short equivalents for images, including icons, buttons, and graphics; Description of data represented on charts, diagrams, and illustrations; Labels for form controls, input, and other user interface components.

  • Operable — Can the user interact with content easily, with a positive outcome? People must be able to operate the interface – the interface cannot require interaction that a user cannot perform. Examples include: All functionality that is available by mouse is also available by keyboard; Keyboard focus does not get trapped in any part of the content.

  • Understandable — Can the user understand the content available to them? People 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. For example: Identifying the primary language of a web page, such as Arabic, Dutch, or Korean; Providing definitions for any unusual words, phrases, idioms, and abbreviations; Using the clearest and simplest language possible, or providing simplified versions.

  • Robust — Does the site or app work consistently with the user's technology available to them? People must be able to access the content as technology advances – as technologies and user agents evolve, the content should remain accessible. Examples of how this can be achieved include: Ensuring markup can be reliably interpreted, for instance by ensuring it is valid; Providing a name, role, and value for non-standard user interface components.

In a nutshell, if any of these principles and guidelines are not met, people with disabilities will not be able to use the web. It's about understanding and meeting the diverse needs of your users.

WCAG Success Criteria

Each of the four principles is made up of several success criteria. These criteria make up the conformance tests you need to conduct in order to create an accessible user experience.

There are three levels of conformance, each increasing in the level of difficulty in order to meet the required success criteria tests. The breakdown of each level is as follows:

  • Level A — Provides a baseline to meet basic usability needs of most people
  • Level AA — Includes usability which meets the needs of most people with disabilities
  • Level AAA — Strict requirements to meet the needs of specific people and may not always be required/depends on the content

For example, under the Operable principle, 2.1.1 Keyboard, which is a Level A requirement, basically states anything someone can do with a mouse also needs to have keyboard support.

The typical level to strive for in order to meet the needs of people with disabilities and to conform with most regional laws is WCAG 2.0 Level AA. This level provides a balance of usability, functionality, and aesthetic design that's achievable by designers and developers.


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