The 3 functions of alternative text:
- Screen readers announce alternative text in place of images, helping users with visual or certain cognitive disabilities perceive the content and function of the images.
- If an image fails to load or the user has blocked images, the browser will present the alternative text visually in place of the image.
- Search engines use alternative text and factor it into their assessment of the page purpose and content.
reference: Alterntive Text, WebAIM