Pinboard: Settings -> Backup -> "HTML (legacy Netscape format almost everyone can read)" Linkding: Settings -> General -> Import "Import bookmarks and tags in

Pinboard: Settings -> Backup -> "HTML (legacy Netscape format almost everyone can read)" Linkding: Settings -> General -> Import "Import bookmarks and tags in the Netscape HTML format. This will execute a sync where new bookmarks are added and existing ones are updated." The stuff in quotes is verbatim from the admin panels, I have not tried it.