Accessibility Basics Part 2: WCAG

In this 2-part article series, we take our popular webinar and presentation “Accessibility Basics” and present it in an easy-to-understand article format. Part 1 begins by explaining what web accessibility is and why it’s needed. In this part, we introduce WCAG.

The Web Content Accessibility Guidelines, or WCAG, were written by accessibility specialists, people with disabilities and software vendors, and published by the Web Accessibility Initiative (WAI) of the World Wide Web Consortium (W3C). The W3C is an international, vendor-neutral organization that works to create standards for the World Wide Web.

The first version of WCAG was published in 1999. Version 2 was published in 2008, and version 2.1 was published in 2018.


WCAG consists of three accessibility conformance levels: Level A (minimum), Level AA (medium) and Level AAA (maximum).

Conformance and conformance levels apply to full web pages only and cannot be achieved if part of the web page is excluded. Only complete processes can be claimed accessible (i.e. all pages in a process must be accessible. For instance, if an online store has an inaccessible credit card form, then the entire site is deemed inaccessible); and only accessibility-supported ways of using technologies are relied upon to claim conformance.

The breakdown

The rules of WCAG are broken up into four principles (Perceivable, Operable, Understandable and Robust), which in turn are broken up into guidelines, then success criteria, then techniques. For instance, the Perceivable principle states that information and user interface components must be presentable to users in ways they can perceive (one implication of this principle is that information cannot be presented in a form that is only available through one sense, such as providing only a visual form of a CAPTCHA).

One guideline of the Perceivable principle (1.1) is defined as:

Provide text alternatives for any non-text content so that it can be changed into other forms people need, such as large print, braille, speech, symbols or simpler language.

One success criterion of this guideline is Success Criterion 1.1.1:

Non-text Content: All non-text content that is presented to the user has a text alternative that serves the equivalent purpose, except for certain situations.

One technique of Success Criterion 1.1.1 is H37: Using alt attributes on img elements.

Techniques fall into one of 12 categories, which determines the letter under which that technique is classified. The categories and their corresponding letters are:

  • General (G)
  • HTML and XHTML (H)
  • CSS (C)
  • Client-side Scripting (SCR)
  • Server-side Scripting (SVR)
  • SMIL (SM)
  • Plain Text (T)
  • Flash (FLASH)
  • Silverlight (SL)
  • PDF (PDF)

In addition there is a Common Failures (F) category which outlines common failures. 


It can be all a little confusing, but the W3C has some great introductory videos and articles that can help:

If you are working on a web project, we have some detailed advice for managers, testers and developers in our Accessibility Factsheets section. We have factsheets on the following categories:

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