WCAG Layers of Guidance
To compose an effective and actionable standard, WCAG created layers so individuals and companies could use as guidance on how to create accessible content online. They include:
Principles
At the highest level, WCAG foundational principles are based on the acronym POUR: Perceivable, Operable, Understandable, and Robust. They provide a clear framework for what it means for content to be accessible.
Guidelines
Under each principle, there is a set of 13 guidelines. They were created to help implement web accessibility, clarify the intent behind each rule, and provide context for the success criteria they are related to.
Success Criteria
Each guideline contains testable success criteria. They are provided to allow WCAG 2.1 to be used where requirements and conformance testing are necessary such as in design specification, purchasing, regulation, and contractual agreements. There are three levels of conformance, designed to meet different needs of different groups.
Sufficient and Advisory Techniques
For each guideline and success criterion in WCAG 2.1, a variety of techniques are provided. These are divided into sufficient techniques, which satisfy the success criteria, and advisory techniques, which offer additional ways to enhance accessibility beyond the basic requirements. Some advisory techniques also address barriers not covered by the testable criteria.
To achieve the highest level of accessibility possible and best address the needs of the widest possible range of users, the recommendation is to apply as many layers as possible, including the advisory techniques.