Jake Archibald, by way of being convinced by Léonie Watson, says that the alt text of an of <img src="..." alt="..." width="..." height="..."> should evoke emotion:
The relevant parts of an image aren’t limited to the cold hard facts. Images can make you feel a particular way, and that’s something that should be made available to a screen reader user.
If necessary, that is. Some images just aren’t particularly emotional, like a screenshot of a banking dashboard. I think what Scott Vandehey says is a more broadly useful way to think about it:
Write alternative text as if you’re describing the image to a friend.
That’s helpful to me. Scott’s article has a bunch more practical advice and links about this topic from the community. Also good to remember: alt text shows when images are broken and different browsers handle that differently depending on how much of it there is.
