WebAuthn Developer Guide
Two-factor authentication (2FA)
The Definitive Guide to 2 Factor Authentication - by Arsen
Web Authentication: An API for accessing Public Key Credentials - Level 2
Facebook two-factor auth
New in Breeze: Two-factor authentication - Breeze
SPRK Reference Guide > Logging In > Two-Factor Authentication
How To Add Two Factor Authentication To Gmail - LogMeOnce Resources
Certified Authenticator Levels - FIDO Alliance