How long can alt-text be?

There are sources on the Web which claim that screen readers, the devices used by blind or visually-impaired people for which alt-text was originally conceived, can only read a maximum of 200 characters of alt-text. Or 125. Or 80.

In reality, even the 200-character limit is an outdated figure from around 2020. Today's screen reader software has no limit for alt-text length, and hardly anyone runs older, limited versions anymore, if at all. This means that the 125-character limit is from some point in the 2010s and even more outdated.

The technical character limit for alt-text in the Fediverse is 1,500. It is imposed by Mastodon, Misskey and their respective forks. Not only don't they allow their own users to post more than 1,500 characters per alt-text, but they also cut incoming longer alt-texts off at the 1,500-character mark and discard everything beyond that mark. So even if you can post longer alt-texts, they will chop them off when they receive them. And unless you're on Hubzilla or (streams), and you've got ActivityPub off, or you have another way of limiting your audience, chances are good that your posts will at least end up on Mastodon.

But even on Friendica, Hubzilla, (streams) and Forte which don't have arbitrarily-chosen alt-text character limits, alt-text cannot be infinitely long. Rather, it is limited by how much alt-text can be shown at must by hovering a mouse cursor above an image. The alt-text in the pop-up cannot be scrolled; any attempt at even only moving the mouse cursor unto the pop-up will close it. The actual limit depends on screen size, screen resolution, font size etc.

Still, very long alt-text is very inconvenient for screen reader users. Normally, screen reader users can navigate through text. They can skip back to any point they have already had read out to have the text re-read from that point. However, screen readers cannot navigate alt-text. They cannot skip back to a point somewhere in the middle of the alt-text. They can only skip back to the beginning of the alt-text and re-read the whole alt-text. The longer alt-text is, the more inconvenient it becomes.

This is the main reason why it's recommended to keep alt-text short and concise. That said, sometimes you don't have any other choice but to make your alt-text longer.