When you visit the web version of iCloud in Safari on an iPhone, iPad, or Mac running the latest beta releases of their respective software, you'll get the option of logging in using biometrics. Currently, Touch ID and Face ID are both usable, though they are limited to certain devices (i.e. only newer iPhone and iPad models support Face ID). Users of older devices, or Macs, are able to log in using Touch ID.
Accessing iCloud.com on one of the aforementioned devices will automatically redirect you to beta.icloud.com where you will be prompted to use either biometrics authentication method to sign into your account. To try this upcoming feature, you need to be running iOS 13, iPadOS 13, or macOS Catalina, and using the Safari browser.
The new functionality was spotted by 9to5Mac, who were able to sign into iCloud using Face ID on an iPhone XS Max running the latest developer beta of iOS 13.
Signing in with Touch ID or Face ID is not only more fast and convenient than entering a (potentially) lengthy password, but it could also be more secure. Apple has previously announced that it will make the "Sign in with Apple" tools available for developers this summer, ahead of iOS 13's official release this fall.