See https://twitter.com/arhayward/status/1367610592807317506.
- Phabricator currently doesn't try to generate default alt text for images, but could at least do something when no human-readable text is available. It's not entirely obvious whether "something" is better than "nothing", though.
- You can specify alt text with {F123, alt="..."}, but File objects could reasonably support alt text as a field for use as a default for contexts where no explicit "alt" attribute is provided.