A screen reader used by a person who is blind can read text with synthesized speech, but it can not read a picture or describe it. Alt text (or alternative text) helps people who can’t see the screen to understand what’s important in images and other visuals. It basically places a text label onto a picture so the screen reader can read the descriptive text.
Important things to consider:
1. In alt text, briefly describe the image and mention the existence of the text and its intent. Include the most important information in the first line, and be as concise as possible.
2. If the object is an image of text, the alt text must match the text verbatim.
3. If the object is a decorative image such as a border, use a space or “double-quote, space, double-quote” as the alt text. This ensures that the screen reader will ignore the graphic.
NOTE: For PowerPoint, it’s recommended only putting text in the description field and leaving the title blank. This will provide the best experience with most major screen readers.