W3 Accessibility Guidelines Core
Non-text Content
All non-text content that is presented to the user has a text alternative that serves the equivalent purpose
The intent of this Success Criterion is to make information conveyed by non-text content accessible through the use of a text alternative. Text alternatives are a primary way for making information accessible because they can be rendered through any sensory modality (for example, visual, auditory or tactile) to match the needs of the user.
W3 Accessibility Guidelines Static Web
When using the img element, specify a short text alternative with the alt attribute. Note. The value of this attribute is referred to as "alt text".
When an image contains words that are important to understanding the content, the alt text should include those words. This will allow the alt text to play the same function on the page as the image. Note that it does not necessarily describe the visual characteristics of the image itself but must convey the same meaning as the image.
Example
An image on a Website provides a link to a free newsletter. The image contains the text "Free newsletter. Get free recipes, news, and more. Learn more." The alt text matches the text in the image.
<img src="newsletter.gif" alt="Free newsletter.
Get free recipes, news, and more. Learn more." />