As you can see in the examples on that page, the ALT tag is short and concise.
This we could debate forever, because it depends on how you populate your title and descriptions. In general, similar to IPTC standards, the title is supposed to be a short concise caption headline, one sentence max (optimally). The description on the other hand, could be several sentences, easily and potentially too long for the alt tag, possibly also considered "keyword stuffing". Also, many users (including myself, and the X3 demo) only require a single caption, which should be the title/headline, leaving the description empty. Furthermore, when using titles AND descriptions, many would write their title/description combo's with title actually describing the image, while description has a story, not necessarily describing the image. Typical scenario:
https://demo.photo.gallery/examples/features/captions/
In either case, it's really not reasonable to assume the description is a better alt tag than title. I also would be skeptical to the amount of text it is wise to add into the alt tag.
metallissimus wrote:In this case I can put my former description in the title because I don't display the title. But if I did display the title, I would need to make some compromise here.
Not sure I understand the dilemma. In X3, you can choose to display title or description or both or none.