Thank you very much, @quid. That is certainly a lot of edits... I only hope it did some good.

Concerning having required tags on the main site, I think that may be difficult to impose given that not all questions allow obvious top level tags. But I also think that we should definitely think about this topic, given that the benefits of guaranteeing every question as some common, descriptive tags would be very significant.

What are your thoughts on that, @quid?

@RicardoAndrade thanks foe the feedback; I agree it couId rather not be just the current toplevel tags. Yet I think with one "not-mathematics" in addition it would already sort of work.

Maybe not really perhaps, because say some might not know that linear-algebra is in ra.rings-and-algebras or they might know but still would find it strange. Yet they could always opt for gm.general-mathematics. Or we could even have a tag "other" (as MSC sometimes has) This tag then could be conveniently monitored.

Or of course we might seize the opportunity to revise the toplevel tags at that point in time.

I do not know about finding crossposts. But there is sme discussion on this on meta.SO but I did not find anything directly useful. But at least on migration there is AFAIK an automatic check. So in principle the infrastructure should be there.