Watsit said:
When I upload posts that have separate descriptions and alt-texts, I place the alt-text underneath the description in a [spoiler] ... [/spoiler], which hides it until the user hovers on it similar to the original alt-text behavior (a [section] ... [/section] requires clicking and takes up more space, which I find a bit annoying for something that's only a couple of lines; it's more work to see, and even the closed section can sometimes end up larger than the text it's holding).

Eh, both sections and spoilers would need to be tapped on mobile, but with sections you can start them expanded