Image Alt Text Guidelines. It should convey the purpose of the image not describe the image. Its original and still primary purpose is to describe images to visitors who are unable to see them.
It should be brief. There is no reason to use any references to image of or graphic image in the alt text as the screen reader will let the user know that the file is an image. Alt text text that describes an image improves accessibility for people who can t see images on web pages including users who use screen readers or have low bandwidth connections.
This includes screen readers and browsers that block images but it also includes users who are sight impaired or otherwise unable to visually identify an image.
When writing alt text follow these guidelines. Its original and still primary purpose is to describe images to visitors who are unable to see them. Energy gov requires alt text to be at least 20 characters long. The alt text shouldn t be longer than a sentence or two most of the time a few thoughtfully selected words will do.