If you want to change the markup then I would target this pr against 4.0 as for me it is a kind of BC break (I know, not covered by the terms), but it would break all sites which are adapted to the old markup.

Well, yes this PR changes the default fieldgroup markup (but not the default single field rendering). Anyway, the current way to render the fieldgroup is wrong and doesn't validate standards.
Is targeting the PR against 3.9-dev acceptable?

Per the B/C strategy:

We will try not to change markup in such a way that a site might render differently, but we can't promise not to break anything at the present time.

In this case it looks like we're dealing with moving an invalid markup structure to a valid one. So I think it'd be acceptable to make that change without waiting for 4.0. But, as it does have potential concerns with page structures, I would suggest it go into 3.9 with a note about the structure change (even if it is relatively minor) versus going into a 3.8 patch release.

Like it is now is to be inline with com_content. For example in com_contact we made a layout override for it to have a similar structure as you propose.
So if this change is going to be accepted a layout override for com_content needs to be done and the one of com_contact adapted.

@laoneo I don't understand why com_content should need a layout override. It just uses the default layout provided by com_fields.

Anyway, I made two mistakes: my previous PR wasn't right and I reworked it. The list.php layout for single field didn't had sense (it was not standard compliant to have a <dt><dd> pair without a <dl> container).
The second mistake is that a <dl> with a single <dd> is not violating standards, so the actual way to render fieldgroups is ok (but I think my PR would make things better).

Because com_content displays the info block with the pattern com_fields has now. If we are going to change that, then it doesn't look like it would nicely integrated.

To make com_content display fieldgroups better, I was planning to add two new layout overrides, unordered list and table. I'm unsure if I should push them to a new PR or if I can add them here.

If changing the default com_fields rendering can be problematic, I can cancel this PR and just add the new overrides to com_content (without changing the default). Maybe this is the best option.

I've just found that my PR it's a duplicate of #17646 , so I'm closing it.

@mattiaverga can you please test #17646?

@franz-wohlkoenig should I test it even if it actually has conflicts?

i wrote now at #17646 to solve Conflicts.