Adding alternative text for images is the first principle of web accessibility. Alternative text provides a textual alternative to non-text content in web pages. This text is the responsibility of web maintainers for each website.
Alternative text serves several functions:
- It is read by screen readers in place of images allowing the content and function of the image to be accessible to those with visual or certain cognitive disabilities.
- It is displayed in place of the image in browsers if the image file is not loaded or when the user has chosen not to view images.
- It provides a semantic meaning and description to images which can be read by search engines or be used to later determine the content of the image from page context alone.
The key principle is that computers and screen readers cannot analyze an image and determine what the image presents. As content creators, text must be provided to the user that presents the CONTENT and FUNCTION of the images within your web content.
Alternative text can be presented in two ways:
- Within the alt attribute of the image component (found in the metadata).
- Within the context or surroundings of the image itself.
The alt-text within the image component should typically:
Be accurate and equivalent in presenting the same content and function of the image.
Be succinct. This means the correct content (if there is content) and function (if there is a function) of the image should be presented as succinctly as is appropriate. Typically no more than a few words are necessary, though rarely a short sentence or two may be appropriate.
NOT be redundant or provide the same information as text within the context of the image.
NOT use the phrases "image of ..." or "graphic of ..." to describe the image. It is usually apparent to the user that it is an image. And if the image is conveying content, it is typically not necessary that the user know that it is an image that is conveying the content, as opposed to text. If the fact that an image is a photograph or illustration, etc. is important content, it may be useful to include this in alternative text.