‘Alt text’ or ‘alt attribute’ refers to text that is attributed to an image to describe the intent of the image rather than a description of what the image is. The ‘alt text’ will display before an image is loaded. If an image fails to load the ‘alt text’ remains visible in its place.
The principle reasons for including ‘alt text’ have to do with web browsers, particularly text-based browsers like Lynx, which never display images at all, and with providing web accessibility for the visually impaired via screen reading devices. Screen readers are electronic readers that scan and read web page content aloud for visually impaired users. For this reason ‘alt text’ needs to be descriptive rather than literal. For example, ‘alt text’ for an image of a dozen red roses in a vase may read: “red roses shipped anywhere.” The ‘alt text’ is meant to reveal not just the image but its significance and relevance to the rest of the content on the web page.
The search engine optimization of ‘alt text’ ensures that the images included on your web page are relevant to its focus and do not in any way dilute its effectiveness. In fact, the keywords and phrases for which the page was optimization should be easily incorporated in the ‘alt text’ for all images you select to accompany your body copy.
