I had a closer look and could effectively only identify one problem.

• There is an invisible tab target right after the close button. It turns out this is the <div class="mw-kartographer-interactive leaflet-container …" tabindex="0"> element that belongs to Leaflet. As far as I can tell this might be relevant for touch devices to be able to move the map with your finger. Not sure.
• The 5 little white rectangles that can be dragged around on the map are all tab targets because of a tabeindex="0". This is also set inside of Leaflet. As above, I think the idea is to make these somehow usable on touch devices.

My impression is that removing these elements from the tab order might break something else, e.g. some touch feature. We would need to dig quite deep into this to be sure. Is this worth it?

I suggest to decline this ticket.

In T308240#7956022, @thiemowmde wrote:

There is an invisible tab target right after the close button. It turns out this is the <div class="mw-kartographer-interactive leaflet-container …" tabindex="0"> element that belongs to Leaflet. As far as I can tell this might be relevant for touch devices to be able to move the map with your finger. Not sure.

Yes, this target is the center of the frame and dragging allows the frame to be moved. The tabindex is not relevant to touch devices, which can be demonstrated by enabling touch in Firefox responsive design mode for example.

• The 5 little white rectangles that can be dragged around on the map are all tab targets because of a tabeindex="0". This is also set inside of Leaflet. As above, I think the idea is to make these somehow usable on touch devices.

At the very least, we should adjust styles so that the frame handles are clearly focused. Currently, it looks like tab focus is completely gone while the user goes through each of these elements. Also, there is no a11y hint for these elements. Again, there seems to be no connection between tabindex and touch devices.