Alt text

Alt text (alternative text) is a short written description attached to an image on a web page. You do not usually see it on the page: a screen reader reads it aloud in place of the image, and a browser displays it if the image fails to load.

Good alt text gives the purpose of the image in that place, not a list of every detail. On a school sports page, a photo of the winning relay team could have the alt text "Year 8 relay team holding the district trophy after the final". Someone who cannot see the photo still knows what it adds to the report. For a chart, "Bar chart: Year 9 earned the most house points in March" is more useful than "a chart". A screen reader already announces that the item is an image, so there is no need to begin with "Image of".

Some images are only decoration, such as a patterned strip beside a heading. These can be given empty alt text (alt="") so a screen reader skips them instead of announcing an image that carries no information. A complex diagram, such as the steps in the water cycle, may need a longer description in the surrounding text, because every label crammed into the alt text is hard to listen to.

Why it matters

A screen reader cannot work out what a picture shows. Without alt text, a blind or low-vision user may hear only "image" and miss the information completely. Alt text also helps any user whose images fail to load, because the description appears in place of the picture. In WCAG (Web Content Accessibility Guidelines), providing text alternatives for non-text content is a guideline under the Perceivable principle: users must be able to receive the content, whatever form it starts in.