Alt Text

Alt text, short for alternative text, is a written description associated with an image so that people who cannot see it still get the information it conveys. It is provided through the alt attribute on an HTML img element, and it is read aloud by screen readers, shown when an image fails to load, and used by search engines. Alt text is one of the most fundamental and most frequently mishandled elements of web accessibility.

In WCAG terms, alt text is governed by success criterion 1.1.1 Non-text Content, which is Level A, the minimum conformance level. That placement reflects how basic the requirement is: an image carrying meaning that has no text alternative is simply unavailable to someone using a screen reader. Because it is a Level A criterion, missing alt text is a failure at every conformance level.

Good alt text describes the image's purpose in context, not merely its literal contents. The same photograph might warrant different alt text on different pages depending on why it is there. Descriptions should be concise, avoid redundant phrases like image of, and convey the function when the image is also a link or button. For complex images such as charts, a short alt value is paired with a longer description elsewhere on the page.

A key nuance is that not every image should have descriptive alt text. Purely decorative images, those that add no information, should carry an empty alt attribute (alt="") so assistive technology skips them rather than announcing a distracting filename. Deciding whether an image is informative or decorative is a judgment call, which is exactly why automated tools can detect a missing alt attribute but cannot verify that the wording is accurate or appropriate.