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.^{1 2 3 4 5} Or 80. Or that the number of characters which they can read in alt-text is generally limited.⁶

In reality, even the 200-character limit is an outdated figure from around 2020.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,^{7 8 9} 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 This means that the 125-character limit is from some point in the 2010s and even more outdated.

There are various alt-text character limits in the Fediverse. On Mastodon, it's 1,500 characters. This means that if you're on Mastodon, you can only write 1,500 characters of alt-text. And if you're someplace that allows for longer alt-texts, and one of your image posts with such a long alt-text ends up on Mastodon, the alt-text is truncated at the 1,500-character mark. Keep in mind that many Mastodon users don't expect anything they see in their timelines to not come from within Mastodon, nor do they expect anything in the Fediverse to have features that Mastodon doesn't have. So they won't necessarily understand that your post comes from outside Mastodon, your alt-text is originally longer at the source, and they should look up your post at the source.

On Misskey, alt-texts are limited to 512 characters. Longer alt-texts should be truncated like on Mastodon. However, Misskey has a bug which deletes longer alt-texts instead of truncating them, and your audience on Misskey will neither know nor suspect that you've written an alt-text. It's likely that this bug is also present on all Misskey forks and forks of Misskey forks: Calckey/Firefish, Iceshrimp (the original Iceshrimp-JS), CherryPick, Sharkey, Meisskey etc.

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 most 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 onto the pop-up will close it. The actual limit depends on screen size, screen resolution, font size etc. You can theoretically write longer alt-texts, but not even you will be able to read them.

Also, 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 cannot pause in the middle of the alt-text either.⁷ 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.^{6 7 10 11} That said, sometimes you don't have any other choice but to make your alt-text longer.