This sounds good, except how would it actually work?
I register in on my iPhone, it uses a key kept on that phone/iCloud. I log in via Safari on MacOS and it works because of iCloud sync.
Now I go to login using Edge on Windows. How can the website find out that I'm the same user as the iPhone/Safari user since I can't sync my key, and I can't enroll my MS Hello ID (or whatever Windows uses) on my Mac or iPhone?