What is alt text?
Alternative text, or "alt text", is a written description of the visual content in an image. It is typically one or two sentences long, and should convey any important visual information. For example, if there is written text in an image, the text content should be replicated in the alt text. You don't need to say "image of" at the beginning of alt text; screen readers will announce that there is an image before reading the alt text.
For example, the following image has the alt text "University of Colorado Boulder logo".
Why is alt text important?
Images are inaccessible to individuals who are blind, unless an image description is provided as alt text.
Will everyone see the alt text?
Alt text is a “behind-the-scenes” description of an image. It’s not typically visible in a document or webpage; it is available to people using a screen reader to allow them to access the content of an image.
What if my alt text is longer than 1-2 sentences?
If a longer description is needed, that description should be provided in the main body of the document or in supplementary documentation instead of alt text. Long descriptions are often needed for complex diagrams such as graphs and flowcharts. If you provide a long description elsewhere, you should still provide alt text (e.g., “diagram of Krebs Cycle; longer description to follow”).
What if my image doesn't contain any meaningful or important content?
If you have an image that is purely meant for visual decoration and contains no meaningful content, you can mark the image as decorative instead of adding alt text. The way to do this depends on your content editor; most software will have a checkbox under image properties that is labeled "Mark image as decorative" or something similar. In HTML, you would add an empty, or null alt attribute (alt="") to the img element. Additionally, if an image contains meaningful content but all of the meaningful content is already provided elsewhere in the text of the document, then you can mark the image as decorative.
How do I write "good" alt text?
Writing alt text can be more of an art than a science. Ask yourself why the image is important in the context of the document, and let that guide your decisions on which details to include in the description. It is best to put the most important idea at the beginning of the alt text. Lastly, avoid providing subjective interpretation; just describe what is visually presented in a neutral tone.