Alt text and image descriptions
Alt text (“alternative text”) is a short sentence that describes an image. Users who are blind or have low vision use screen reader software to explore websites and rely on these descriptions to perceive and understand visual web content. Alt text is not usually visible on your webpage by default, but screen readers scan the web page’s code and relay alt text lines to the user. An image description (“ID”) works similarly, but is longer and describes the image in greater detail. Both of these elements are distinct from an image caption, which usually sits below the image and is both captured by screen readers and visible at all times.
If you have the time and resources, including both alt text and an image description is always better. In that case, the alt text serves as a title for the image, and the image description conveys greater detail without repeating the alt text. However, at the absolute minimum, all images on your site should have alt text lines populated with meaningful text (not the name of the image file, “img02.jpg” which sometimes automatically gets put there when you upload the picture).
The image below demonstrates an example of both alt text and image descriptions.
Alt text and image descriptions normally aren’t visible on a website, but here’s what they might look like for the image above:
Alt text: “Sue,” a Tyrannosaurus Rex skeleton located in the Field Museum of Natural History, Chicago, Illinois.
Image description: Sue’s mouth is slightly open, and their right leg is forward and slightly raised, as if the dinosaur were walking. People stand around Sue, looking at the skeleton. Behind the fossil, on the same floor, is an entrance to the museum, and a green sign with the word “EXIT” in white letters stands in the distance. On the floor above, a semi-circular mural shows an artist’s imagining of a Tyrannosaurus Rex during its lifetime and in its natural habitat.
Note that if you choose to do only an image description, you should also include the alt text as the first sentence.
Writing alt text and image descriptions
The University of Dayton has published an informative blog post about how to write effective alt text. If you know nothing about the topic, they offer a good starting point.
For information on writing good image descriptions, check out this article by the American Anthropological Association. Alex Chen has also written an important guide on the object-action-context model of image descriptions. This method makes for consistent and comprehensible descriptions.
While writing alt-text is frequently a do-it-yourself project, third-party paid services like Scribely may help with large-scale projects that call for many images.
Captions transcribe the audio of a video for users who are d/Deaf or hard of hearing. They are also useful for those who choose not to turn on a video’s sound, such as people on a public bus or train. In most cases, videos contain closed captions, which are encoded separately so a user can turn them on or off. Closed captions are generally better than open captions, which are permanently part of the video file, because screen readers can more effectively read closed captions. Note that captions are usually distinct from subtitles: text translated into a different language.
YouTube includes captioning options—both manually- and automatically-generated. Manually-created captions are ideal, but they can be labor-intensive. While automatic captions can help save time and labor, they can also contain errors. For this reason, it’s important to manually review and edit any automatically-generated captions to make sure your content is clear. The video below provides a handy guide to creating and reviewing automatic captions in YouTube.
If your website uses embedded video, consider uploading that material to your site through Able Player, a free and open-source media player that strives to be fully accessible. While Able Player requires manually-transcribed captions, tools such as WGBH-TV’s free CADET software make crafting them easy. If you have budget room, groups like 3Play Media and Georgia Tech’s Center for Inclusive Design and Innovation also offer paid captioning services.
Audio descriptions are similar to image descriptions (see above). However, they outline videos rather than static images. These help make videos more accessible to users who are blind or who have low vision.
The Audio Description Project has become the premier site for audio description news, best practices, and services. Its comprehensive scope makes it a key resource on audio descriptions.
You can create audio descriptions through a process similar to making captions. The caption-writing resources listed above above (both free and paid) can help develop them. Able Player also supports audio descriptions.