Currently, for image assets, both the “Title/Caption” and “Alt Text” are plaintext fields:
Can Title/Caption allow rich text? It can be used to populate <figcaption> tags, HTML flow elements (including em and strong should be allowed).
This is just so you can apply basic formatting to image captions, such as bolding or italicizing certain words or adding links inside of figcaption.
The alt text should remain plaintext.