That reminds me.. On macOS, the default application to open files with the .html extension is Safari, which I found is impossible to change. A tiny thing but it still makes me angry that someone at Apple intentionally removed my ability as a user.
Apparently this was an issue with the user, not the OS. My bad.
EDIT: Actually, I remember now that I had to use duti to change this from the command line.
Do you have some more context? I have found this trivial to change...