Q:

Are questions about usability and user interface on topic?

I just saw this question. It's tagged with usability and user-interface because it's asking about user interface design (in this case in html but it's pretty platform/language agnostic).
Questions about usability are not strictly about programming and they have their natural home in another StackExchange site: User Experience. Should they be considered off-topic when they're only about design without a specific programming issue (as, for example, stated on this post)?
On the other side we have 790 questions tagged usability (user-interface is too generic to be considered on- or off-topic when used alone).
Are these questions on topic for StackOverflow or - in theory - they should all closed/deleted and/or migrated to UX? What we should do with newer questions? Is usability a red flag for possibly off-topic questions?

A:

Most new questions about usability and UX should be asked on UX. However, the question could be on topic here if, for example, it's asking something along the lines of:

I'm trying to implement this usability feature and I'm getting this error/behaviour.

as this is an implementation question which would be off topic over there. So while you might want to take a closer look at questions tagged usability the presence of the tag is not sufficient to indicate that the question should be migrated.
Any old* question can't be migrated, so if it's now off topic then it should be closed. However, I wouldn't go searching for them, but only vote to close if you came across them while looking for something else.
* old is defined as older than 60 days.

