The Web Content Accessibility Guidelines (WCAG) are part of a series of web accessibility guidelines published by the Web Accessibility Initiative (WAI) of the World Wide Web Consortium (W3C), the main international standards organization for the Internet. They are a set of recommendations for making Web content more accessible, primarily for people with disabilities—but also for all user agents, including highly limited devices, such as mobile phones. WCAG 2.0, was published in December 2008 and became an ISO standard, ISO/IEC 40500:2012 in October 2012. WCAG 2.1 became a W3C Recommendation in June 2018.
The first web accessibility guideline was compiled by Gregg Vanderheiden and released in January 1995, just after the 1994 Second International Conference on the World-Wide Web (WWW II) in Chicago (where Tim Berners-Lee first mentioned disability access in a keynote speech after seeing a pre-conference workshop on accessibility led by Mike Paciello).
Over 38 different Web access guidelines followed from various authors and organizations over the next few years. These were brought together in the Unified Web Site Accessibility Guidelines compiled at the University of Wisconsin–Madison. Version 8 of the Unified Web Site Accessibility Guidelines, published in 1998, served as the starting point for the W3C's WCAG 1.0.
The WCAG 1.0 was published and became a W3C recommendation on 5 May 1999. They have since been superseded by WCAG 2.0.
WCAG 1.0 consist of 14 guidelines—each of which describes a general principle of accessible design. Each guideline covers a basic theme of web accessibility and is associated with one or more checkpoints that describes how to apply that guideline to particular webpage features.
Each of the in total 65 WCAG 1.0 checkpoints has an assigned priority level based on the checkpoint's impact on accessibility:
WCAG 2.0 was published as a W3C Recommendation on 11 December 2008.  It consists of twelve guidelines (untestable) organized under four principles (websites must be perceivable, operable, understandable, and robust). Each guideline has testable success criteria (61 in all). The W3C's Techniques for WCAG 2.0 is a list of techniques that help authors meet the guidelines and success criteria. The techniques are periodically updated whereas the principles, guidelines and success criteria are stable and do not change.
Information and user interface components must be presentable to users in ways they can perceive.
User interface components and navigation must be operable.
Information and the operation of user interface must be understandable.
Content must be robust enough that it can be interpreted reliably by a wide variety of user agents, including assistive technologies.
WCAG 2.0 uses the same three levels of conformance (A, AA, AAA) as WCAG 1.0, but has redefined them. The WCAG working group maintains an extensive list of web accessibility techniques and common failure cases for WCAG 2.0.
The first concept proposal of WCAG 2.0 was published on 25 January 2001. In the following years new versions were published intended to solicit feedback from accessibility experts and members of the disability community. On 27 April 2006 a "Last Call Working Draft" was published. Due to the many amendments that were necessary, WCAG 2.0 was published again as a concept proposal on 17 May 2007, followed by a second "Last Call Working Draft" on 11 December 2007. In April 2008 the guidelines became a "Candidate Recommendation". On 3 November 2008 the guidelines became a "Proposed Recommendation". WCAG 2.0 was published as a W3C Recommendation on 11 December 2008.
A comparison of WCAG 1.0 checkpoints and WCAG 2.0 success criteria is available.
In early 2014, WCAG 2.0's Level A and Level AA success criteria were incorporated as references in clause 9.2 ("Web content requirements") of the European standard EN 301 549 published by ETSI. EN 301 549 was produced in response to a mandate that the European Commission gave to the three official European standardisation bodies (CEN, CENELEC and ETSI) and is the first European standard for ICT products and services.
...initiated with the goal to improve accessibility guidance for three major groups: users with cognitive or learning disabilities, users with low vision, and users with disabilities on mobile devices
and is backwards-compatible with WCAG 2.0, which it extends with a further 17 success criteria.
Businesses that have an online presence should provide accessibility to disabled users. Not only are there ethical and commercial justifications for implementing the Web Content Accessibility Guidelines, in some countries and jurisdictions, there are also legal reasons. Under UK law, if a business's website is not accessible, then the website owner could be sued for discrimination.
In January 2017, the U.S. Access Board approved a final rule to update Section 508 of the Rehabilitation Act of 1973. The new rule adopts seventeen WCAG 2.0 success criteria, but 22 of the 38 existing A-level and AA-level criteria were already covered by existing Section 508 guidelines. The rule requires adherence to the new standards twelve months from its date of publication in the federal register.  
In 2017, a Federal Court in Florida identified the WCAG guidelines as the "industry standard" for website accessibility and found that Winn Dixie Store, Inc., violated the Americans with Disabilities Act by failing to render its website accessible to the sight impaired.
Directive 2016/2102 requires websites and mobile applications of public sector bodies to conform with WCAG 2.1 Level AA. New websites must comply from 23 September 2019 on, old websites from 23 September 2020 on and mobile applications from 23 June 2021 on. The European Parliament has approved the directive in October 2016, the European Commission updated the WCAG reference from 2.0 to 2.1 in December 2018.
In January 2012, the Royal National Institute for the Blind (RNIB) in the United Kingdom issued a press release stating that it had served legal proceedings against low-cost airline Bmibaby over their "failure to ensure web access for blind and partially sighted customers". As of October 2011, at least two actions against websites had been initiated by the RNIB, and settled without the cases being heard by a court.
The Israeli Ministry of Justice published regulations in early 2014, requiring Internet websites to comply with Israeli Standard 5568, which is based on the W3C Web Content Accessibility Guidelines 2.0.
The main differences between the Israeli standard and the W3C standard concern the requirements to provide captions and texts for audio and video media. The Israeli standards are somewhat more lenient, reflecting the current technical difficulties in providing such captions and texts in Hebrew.