Web accessibility, often abbreviated "a11y" (a numeronym formed by "a" + 11 letters + "y"), refers to the practice of designing and developing websites so that people with disabilities can perceive, understand, navigate, and interact with them. This is related to but distinct from usability, which is the broader quality of a design being easy to use for everyone, including those without disabilities. Accessibility specifically targets barriers that disabled users encounter, while usability addresses general ease-of-use concerns across the entire audience.
The Web Content Accessibility Guidelines (WCAG) provide the foundational standards for web accessibility. WCAG 2.1 primarily addresses four disability categories: visual, auditory, physical/motor, and cognitive/neurological. The guidelines are organized around four principles, commonly remembered by the acronym POUR: content must be Perceivable, Operable, Understandable, and Robust. These principles structure all subsequent success criteria that web authors must satisfy.
WCAG defines three conformance levels: A (the lowest), AA (mid-level), and AAA (the highest), each imposing progressively stricter requirements. For color contrast, which is essential for users with low vision, WCAG specifies minimum ratios between text and its background. At level AA, normal-sized text requires a contrast ratio of at least 4.5:1, while large text — defined as 18 point or larger, or 14 point or larger when bold — requires only 3:1 because it is easier to read. At level AAA, normal text demands an even stronger ratio of 7:1 to accommodate users with more severe visual impairments.