What is alt text?

Alternative text (alt text) is text that describes the content of an image that can be read by a screen reader. It's important to consider what elements of the image are important to include in context. A portrait photo of a historical figure may just need that figure's name as the alt text, while a piece of art may need its primary features denoted, and a diagram or graph may need its contents fully described.

What needs alt text?

All images that are used in an instructional or additive manner should be given alt text. Irrelevant images should be marked as decorative (or tagged as background/artifact in .pdf). While "decorative" is the main case in which an image should be ignored, other use cases may also necessitate an image being marked as decorative. An image may convey information, but it may be already stated elsewhere. If the image adds no additional perspective, that image should be marked as decorative. Another example may be an image that's repeated on every page. It may even have an informative role, but in that case the first occurrence should be given alt text, and the repeated instances should be decorative.

General alt text guidelines

  • Alt text should be as concise as possible while still fully describing the important parts of the image. Screen readers read it linearly, so unlike other text, the user cannot jump back to reread portions without reading the entire alt text again.
  • It should not include text like "image of", as that is already provided by the screen reader and would be redundant.
  • It should not include text that is in the caption or repeated elsewhere on the page.
  • It should not provide additional information that is not present in the image itself. For example, if you're alt texting a photo of the moon landing, you may describe the lander, the flag, and Buzz Aldrin, but you shouldn't include the date or that they launched on a Saturn V.
  • Include any text that's present in the image in your alt text, unless it's an unimportant background feature.

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