Yesterday, I posted a video of a 15th century Mass. I said, 'I've no idea if this is an actual Mass, or simply a historical recreation by laymen, but it's fascinating, nonetheless.
However, today, after the point was discussed on an FB group, I found this:
This video was made together with the Parish, Kristi Lekamens Katolska församling i Visby, and sung in one of the mediaeval churches of the Island of Gotland. Anders Piltz, the Priest is a professor of Latin in Lund University and also a Catholic Priest. The Cantor is Mattias Östborn also Cantor at the Catholic church of Visby.
So, it was indeed a valid Catholic Mass, celebrated in a Church stolen from us at the Deformation!