How do you write effective alt text for books?

Alt text, the alternative text attribute of an HTML img element, is one of the most visible and most frequently mishandled accessibility requirements for ebooks. When a screen reader encounters an image, it reads the alt text aloud. If the alt text is absent, the reader hears the filename. If it is present but poor, the reader receives no useful information. Writing good alt text requires understanding what the image communicates and expressing that accurately in words.

The first decision is whether an image is meaningful or decorative. A meaningful image conveys information that is not available in the surrounding text: a photograph of a person, a chart showing data, a diagram explaining a process, a map indicating locations. Meaningful images must have descriptive alt text. A decorative image adds visual interest but conveys no information: an ornamental border, a background texture, a stylistic divider. Decorative images must have an empty alt attribute (alt="") so that screen readers skip them entirely. Using a filename or the word "image" as alt text for a decorative image is worse than an empty alt because the screen reader announces irrelevant content.

For simple meaningful images such as a photograph, a portrait, or an illustration, alt text should describe what the image shows in terms relevant to the surrounding content. It does not need to describe every visual detail; it needs to convey the information the image is meant to communicate. A photograph of a city skyline used to illustrate a chapter on urban development might have alt text: "Aerial view of a dense urban centre with high-rise office buildings and a river running through the middle." The length of the alt text should match the complexity of the image; a short, accurate description is better than a long, wandering one.

Complex images such as charts, graphs, diagrams, maps, and technical figures require extended descriptions. A bar chart comparing sales figures across five regions cannot be adequately described in a sentence or two. The extended description should convey the data or information the chart shows rather than describe its visual appearance. For a bar chart, this means stating the values, the trend, or the key finding. Extended descriptions can be provided as a visible caption below the image, as a details element adjacent to the image, or as a linked long description. The approach must be consistent and clearly indicated.

Mathematical and scientific notation requires special handling. Equations presented as images are not accessible regardless of how detailed the alt text is, because braille math translators and screen reader math modes cannot process image-encoded equations. MathML is required for all mathematical expressions. Where equations are currently stored as images, a common situation in legacy content, remediation requires recreating them in MathML, which is a significant but necessary undertaking for educational and scientific publishers.

Alt text must be reviewed by a human editor who understands the content. Automated tools can identify missing alt text but cannot assess whether existing alt text is accurate or sufficient. The discipline of alt text review is a skill that develops over time; publishers who train their editorial teams to review alt text systematically produce consistently better outcomes than those who treat it as a production afterthought.

The approved copy also has to cover AI-generated alt text for books, each of which a publisher expects answered plainly rather than named in passing.

Nothing on this page has been reviewed. Its provenance is recorded as unverified in the content ledger until somebody signs it off.

The approved copy also has to cover AI-generated alt text for books, each of which a publisher expects answered plainly rather than named in passing.

Nothing on this page has been reviewed. Its provenance is recorded as unverified in the content ledger until somebody signs it off.

The approved copy also has to cover AI-generated alt text for books, each of which a publisher expects answered plainly rather than named in passing.

Nothing on this page has been reviewed. Its provenance is recorded as unverified in the content ledger until somebody signs it off.

The approved copy also has to cover AI-generated alt text for books, each of which a publisher expects answered plainly rather than named in passing.

Nothing on this page has been reviewed. Its provenance is recorded as unverified in the content ledger until somebody signs it off.

The approved copy also has to cover AI-generated alt text for books, each of which a publisher expects answered plainly rather than named in passing.

Nothing on this page has been reviewed. Its provenance is recorded as unverified in the content ledger until somebody signs it off.