Web Authentication: An API for accessing Public Key Credentials - Level 2
What is 2FA? A simplified guide to two-factor authentication - Norton
A Guide to Common Types of Two-Factor Authentication on the Web
Two-factor authentication explained: How to choose the right level of security for every account
Two-Factor Authentication for Beginners, by Martin Shelton
Certified Authenticator Levels - FIDO Alliance
Duo Universal Prompt - Guide to Two-Factor Authentication · Duo Security
Using Google Authenticator for 2 factor authentication
Types of two-factor authentication, pros and cons: SMS, authenticator apps, YubiKey
Authentification Guide — Two authenticators