What does "caption" mean, and is it the same as alt-text?

Some alt-text guides on the Web mention "captions" as a place where image descriptions can go.

What captions are in comparison with alt-texts

But for starters, a caption is something else than an alt-text.

An alt-text is a short and concise image description that hides behind the image. It is not visible to the naked eye, and it is not meant for people who can see the image. It's a stand-in for when the image cannot be seen, either because the person coming across the image is unable to see the image due to a disability or because technological reasons prevent the image from being shown.

Some people cannot access alt-texts, so alt-text must never contain explanations or other additional information that's neither in the message text nor visible in the image to sighted people. Mastodon users tend to handle this differently, but they're wrong.

Mastodon's alt-text button is a first. It's an offspring of Mastodon's particular alt-text culture, and it's unique on the World-Wide Web.

In HTML, alt-texts are part of the image-embedding code:

<img src="ImageFileURL" alt="Alt-text" />

A caption is a longer image description that is rendered below the image. It is always visible, and on websites and in blog posts that can be written in HTML, it is the primary place for image explanations.

In HTML, captions are attached to embedded images by wrapping both into a figure:

<⁠figure><img src="ImageFileURL" alt="Alt-text" /><⁠figcaption>Caption<⁠/figcaption><⁠/figure>

Captions don't exist in the Fediverse

In the Fediverse, none of the above really matters. It only serves to explain to you what a caption is. For captions don't exist in the Fediverse.

Server applications like Mastodon may have a dedicated alt-text field. But they do not have a caption field.

Even server software like (streams) or Forte, both of which accept HTML for formatting posts in addition to Markdown and extended BBcode, have no support for the figcaption tag. Trust me, I've tested this on (streams) myself.

Besides, even if they did have support for the figcaption tag, it'd be pretty futile. Mastodon, the one place in the Fediverse where accessibility matters most, doesn't even support rendering figcaption captions. Even if you managed to add them, Mastodon's HTML "sanitiser" would go and purge them. You'd be able to add them, but those for whom they'd be the most important wouldn't be able to see them.

If you really need to explain your image, do it in the message text.