A detailed #ImageDescription of a complex image does not go into the #AltText. The #W3C says so:

https://www.w3.org/WAI/tutorials/images/complex/

If it can be useful for sighted people, or if everyone, sighted people included, needs it to understand the picture, the image description goes elsewhere. That way, everyone can access it, so it's a matter of #accessibility.

The W3C recommends to put it on a separate page. But the W3C implies that everything on the Web is static webpages accessed via standard desktop Web browsers. That's what the W3C is all about.

Obviously, this doesn't work in the #Fediverse. Unless you're on #Hubzilla, you can't put #ImageDescriptions on separate pages because you can't make such pages in the first place. So the image description has to go into the post.

Again, no problem on Hubzilla which has virtually no post length limit. Same for #Friendica and #Streams. Depending on how long your description is, most other projects may not be problematic either.

But then there's #Mastodon. Only one project, but it's 80% of the Fediverse, and many Mastodon users think it is the Fediverse, full stop.

Mastodon limits posts to only 500 characters. If you came over from #𝕏 where you only had 280 characters, that sounds like a whole lot. But in practice, it's way too little, especially for image descriptions. This, by the way, is why "image description" and "alt-text" are synonymous and mutually exchangeable on Mastodon: Everyone puts their image descriptions into the alt-text which offers 1,500 characters. People can't even imagine that it could be any other way.

But it has to, also because any information available only in the alt-text is lost to those who can't access alt-text, for example due to a physical disability.

So on Mastodon with its 500-character limit, detailed image descriptions should go into a thread that follows the post with the image.

In either case, there should still be an alt-text that says that the image is described in follow-up posts (Mastodon) or the post itself (everywhere else).