Adding alt text on Mastodon has been around roughly as long as Twitter. It took about 10 months to roll out between 2016 and 2017. What I love about the issue discussion is that once the matter was raised, nobody argued against it.
https://github.com/mastodon/mastodon/issues/117
One of the joys of open source software is that the user gets to participate in and have a window into the development process.
More recently...there is an on going discussion over whether or not to introduce Twitter's ALT button overlay to Mastodon.
https://github.com/mastodon/mastodon/issues/18394
One of the commenters linked to their own evaluation of the ALT feature on Twitter:
https://adrianroselli.com/2022/04/keyboard-challenges-for-twitters-new-alt-badge.html
A good read.
Anyway, taking inspiration from those discussions I've injected some CSS (https://gist.github.com/FiXato/3de505b04efefd49a1e3568b70545234) to a11y.social that surrounds images with a dotted red border if they DO NOT have alt text.
As a sighted user, I found Twitter's ALT button useful for deciding whether or not to offer up a description in a reply.
Here is a quick test. If you are browsing Mastodon from the a11y.social instance you should see a red border around the image attached to this post. If you are on a different instance, tell your admin to add the CSS linked in my previous toot!
Alt text (intentionally excluded from image): a German Shepherd asleep on a couch with a tennis ball in its mouth.
Interesting. So because I edited the first post, an empty alt attribute was added to the image which breaks the highlighting rule in CSS.
