In the alt-text case specifically, you could allow for optional styling info so that the gloss can be laid out as a visual replacement for the original text that was on the page. But that's not really necessary, and might even be counterproductive to some use-cases (like when interpretation of the meaning of the text depends on details of typography/calligraphy that can't be conveyed by the gloss, and so the user needs to see the original text with the gloss side-by-side; or when the gloss is a translation and the original is written with poetic meter, such that the user wants the gloss for understanding the words but the original for appreciating the poesy of the work.)
Concrete use-cases:
• the "cleaner" and "layout" roles in the (digitally-distributed) manga localization process, only continue to exist, because soft-subbed images (as standalone documents) aren't a thing. Nobody who has any respect for art wants to be "destructively restoring" an artist's original work and vision, just to translate some text within that work. They'd much rather be able to just hand you the original work, untouched, with some translation "sticky notes" on top, that you can toggle on and off.
• in the case of webcomic images that have a textual "bonus joke" (e.g. XKCD, Dinosaur Comics), where this is currently implemented as alt/title-attribute text — this could be moved into the image itself as a whole-image annotation, such that the "bonus joke" would be archivally preserved alongside the image document.