When I went back tried again - this time the "Add to Chrome" button changed the "Add to Vivaldi" and it did work! The same link also worked with Opera.🙌
I use Safari most of the time with Brave & Opera as my a backup browsers when Safari fails. Some people mentioned liking Vivaldi so I thought I'd give it a try. I haven't found how to stop the annoying operation where Vivaldi opens in full screen mode on a separate desktop. It doesn't do this on my family laptop. Firefox got uninstalled most places - due to their stance on Ai and weaselyness about selling user data.
Thanks for the reply - I don't understand why it didn't work before 🤷♂️