Glossary

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ARIA(also: Accessible Rich Internet Applications, WAI-ARIA)

A W3C specification that provides a framework of roles, states, and properties to make dynamic web content and custom user interface controls accessible to assistive technologies. ARIA supplements HTML semantics where native elements are insufficient. The first rule of ARIA is:…

Adaptive content(also: Content adaptation, Adaptive hypermedia)

Content that automatically adjusts its presentation, structure, or level of detail based on the needs, preferences, or capabilities of the user or their device. In accessibility, adaptive content goes beyond responsive design (which adjusts layout) to potentially restructure…

Alt text(also: Alternative text, Text alternative)

A textual description of non-text content (primarily images) provided via the HTML alt attribute, read by screen readers and displayed when images cannot be loaded. Required by WCAG Success Criterion 1.1.1 (Level A), alt text should convey the same information or function as the…

Audio description(also: AD, Described video, Video description)

Narration added to a media soundtrack that describes important visual information — such as actions, characters, scene changes, and on-screen text — that cannot be understood from the main audio alone. Audio description makes video and live performance content accessible to…

Captioning(also: Captions, Closed captions, Subtitles for the deaf and hard of hearing)

The conversion of spoken dialogue, sound effects, and other auditory information into synchronised text displayed alongside audiovisual content. Captioning makes audio content accessible to people who are deaf or hard of hearing and also benefits people in noisy environments,…

Colour blindness(also: Color blindness, Colour vision deficiency, CVD)

A condition affecting the perception of colour, caused by absent or altered photoreceptors in the retina. The main types are classified by which colour receptors are affected: protanopia (absent red receptors), deuteranopia (absent green receptors), and tritanopia (absent blue…

Colour contrast ratio(also: Contrast ratio, Luminance contrast ratio, Color contrast)

A numerical measure of the perceived brightness difference between a foreground colour (typically text) and its background, expressed as a ratio ranging from 1:1 (no contrast, identical colours) to 21:1 (maximum contrast, black on white). WCAG defines minimum contrast ratios to…

Data visualization accessibility(also: Chart accessibility, Accessible data visualizations, Graph accessibility)

The practice of making charts, graphs, maps, dashboards, and other visual representations of data perceivable and understandable by people with disabilities, particularly screen-reader users and people with colour vision deficiencies. Most web-based visualizations are rendered…

Dexter Model(also: Dexter Hypertext Reference Model)

A reference model for hypertext systems published in 1988 that defines the fundamental concepts of components, links, anchors, and presentation specifications. The Dexter Model was created as a superset description of what hypertext systems could be, with individual systems like…

Hypermedia

An extension of hypertext that incorporates multimedia content — audio, video, images, and animation — alongside text, all linked in a non-linear structure. What distinguishes hypermedia from hypertext is the implied sequencing required for time-based media: a video contains…

Hypertext

A way of organising and presenting textual content in a non-linear manner by breaking it into fragments and describing the relationships between them. A book has chapters in a fixed order, but hypertext allows content to be navigated in multiple paths through links between…

Image description(also: Long description, Extended image description, Image alt)

A detailed textual representation of an image's content, typically longer and more comprehensive than alt text. While alt text is a concise attribute embedded in HTML for screen readers, image descriptions may appear as visible captions, be provided via the longdesc attribute or…

Photosensitive epilepsy(also: PSE, Photosensitivity, Visually-provoked epilepsy)

A form of epilepsy in which seizures are triggered by visual stimuli such as flashing lights, strobing effects, rapid colour transitions, or high-contrast repeating patterns. It affects approximately 3 to 5 percent of people with epilepsy and is more common in children and…

SVG(also: Scalable Vector Graphics)

An XML-based markup language for describing two-dimensional vector graphics, widely used on the web for icons, illustrations, data visualizations, and interactive graphics. Unlike raster images (JPEG, PNG), SVGs are composed of DOM elements that can be individually styled,…

Semantic HTML

The practice of using HTML elements according to their intended meaning rather than their visual appearance. Semantic elements like <nav>, <main>, <article>, <aside>, and <header> communicate the structure and purpose of content to browsers, assistive technologies, and search…

Variable Font(also: OpenType Variable Font, Parametric Font)

A font file that contains multiple design variations along one or more continuous axes — weight, width, slant, optical size, or custom axes — allowing any intermediate value to be rendered at run time. Defined in the OpenType 1.8 specification (2016), variable fonts reduce file…

Virtual environment(also: Virtual world, VR environment, 3D virtual space)

A computer-generated simulation of a three-dimensional space that users can navigate and interact with, typically rendered visually through screens or head-mounted displays. Virtual environments range from simple 3D spaces to complex simulated worlds used in gaming, training,…

W3C(also: World Wide Web Consortium)

The main international standards organisation for the World Wide Web, founded by Tim Berners-Lee in 1994. The W3C develops and maintains specifications including HTML, CSS, SVG, and ARIA. Its Web Accessibility Initiative (WAI) is responsible for the Web Content Accessibility…

WCAG(also: Web Content Accessibility Guidelines, WCAG 1.0, WCAG 2.0)

A set of guidelines published by the W3C Web Accessibility Initiative (WAI) that define how to make web content more accessible to people with disabilities. WCAG is organized around four principles: Perceivable, Operable, Understandable, and Robust (POUR). The current version in…

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