Someone asked me about limits last year, and that must be where they got the idea from. FWIW, at the time (June last year), LinkedIn only allowed 300 chars where it now allows 1,000. I haven't tried dropping an entire Wikipedia page in Facebook's alt text today, but based on my result from last year, if there is a limit there (lower than 42k), it will be on the Facbeook side.

@NVAccess and re: your mastodon comment, yeah, every mastodon server can set different limits. Just like every other website, hence the question for me as a website operator: do I set a limit.

The specific use case for me is online scientific tutorial content, where the images can be quite complex

So, having real hard evidence that there is no limit is extremely useful to know and I can stop thinking about how best to support long descriptions.

I guess the question is how will having (or not) a limit affect your users? It may encourage people to be more succint - and alt text is supposed to be succinct. But whatever limit you set, you will then potentially have people say "But I need more room to describe this really complex chart!". I haven't seen many complaints about limits of around a thousand characters. It's double the average Mastodon post lengh & 4 times a tweet so that might be a place to start if you want a limit.

@NVAccess yeah given what you and others have said, we will not impose a limit.

Conciseness is good, but getting scientists to write ANY alt text is not a fun experience, so, it is unlikely they would even reach 1k characters.

It was /only/ going to be necessary if some screenreaders would just stop announcing, then we'd probably still support longer but have to write some complicated workarounds to ensure that it got chopped up into smaller blocks when too long, or announced as "follow the next link for the long textual description of this image" which would let the user navigate to a text block and then somehow back.

That was a development activity I was not especially looking forward, and now I can just mark the issue resolved and move on! Less work for me, I'm happy.

& from our side - getting some web developers to write even vaguely accessible code sounds like a similar exercise to getting your scientists to write alt text (big generalisation I know!) - so for someone like you who IS doing the right thing, the last thing we want to do is make you do MORE work. If there was a limit from the screen-reader side, then I'd definitely encourage you to contact us (if it's in NVDA at least) as that is something we should be looking to fix rather than you.