I'm in complete agreement about usernames, but if you're at the point where you want to use Unicode in a password, you might as well make the jump to WebAuthn.

I'm in complete agreement about usernames, but if you're at the point where you want to use Unicode in a password, you might as well make the jump to WebAuthn. Going from a UTF-8 input to a normalized bitstream that gets fed into a KDF could be tricky.