Two rules of thumb I use when writing alt text for images are these:
- If you were to describe the document to someone over the phone, would you mention the image or its content? If you would, the image probably needs an alternative text. If not, it should probably have an empty alt attribute.
- Does the alternative text of any images in the document make sense if you turn off the display of images in your web browser? If not, change the alternative text so it does make sense and does not provide redundant information.
These two rules should help anyone determine if alt text is necessary and to write good alt attribute text. By these rules, most images won’t need any alt text and it should be left empty. In certain cases (perhaps more often for sites who focus more on visual content) the image is the focus and proper alt text to describe the image in order to make sense of the document is required.