Alternative text (alt text)

Alternative text — commonly "alt text" — is a concise textual description attached to a non-text element so that the information or function the element conveys is available to people who cannot see it. In an accessible PDF, alternative text is stored as the Alt entry on the structure element (typically a Figure tag) that wraps an image, and it is exposed to assistive technology such as screen readers and braille displays, which read it aloud or render it in braille in place of the image.

Alt text is the primary mechanism for satisfying the requirement that all meaningful non-text content have a text alternative — a principle expressed in WCAG success criterion 1.1.1 (Non-text Content) and mirrored in PDF/UA (ISO 14289), which requires that meaningful graphics carry alternate descriptions and that purely decorative content be marked so that it is skipped. The purpose of alt text is to convey the same information or purpose as the visual element, not to describe its appearance. A good alternative describes what the image communicates in context: for a chart, the key data or conclusion it presents; for a photograph that illustrates a point, the relevant meaning rather than an inventory of visual details; for a functional image such as an icon acting as a button or a linked logo, the action or destination it represents.

Different categories of non-text content call for different treatment. Meaningful images require a genuine text alternative. Complex images such as detailed diagrams, infographics, or data charts may need a short alt text plus a longer description elsewhere so the full information is available. Images of text should generally be avoided, but where present, the alternative must include the words shown. Decorative images — those that add only visual styling — should be given an empty/null alternative or marked as an artifact, so assistive technology skips them rather than announcing an unlabelled graphic.

Alt text matters because images, charts, logos, and other graphics are frequently where critical information lives, and without a text alternative that information is simply lost; an image with no alternative is often announced only as "graphic" or by its filename. Because whether a description is accurate and sufficient depends on the author's intent and the surrounding context, the quality of alt text is inherently a matter of human judgement even though its presence or absence can be detected automatically.