If you are running the beta build of iOS 13, iPadOS 13, or macOS Catalina, you can sign into iCloud on the web using Face ID or Touch ID. This is a new login procedure that Apple is testing as a part of iOS 13/iPadOS 13/macOS Catalina.
When users running iOS 13 or macOS Catalina beta go to iCloud.com, they would be automatically redirected to beta.iCloud.com. They will then be shown a prompt where they can confirm if want to log into their iCloud account using Face ID/Touch ID. It goes without saying that on devices where Face ID is available, the site will rely on that, while on older devices, users will have to use Touch ID.
In case you don’t want to log in with your primary iCloud ID, there is an option to log in with a different Apple ID.
This is definitely a convenient feature as it would make logging into iCloud services notably easier. Apple is testing this feature as a part of ‘Sign in with Apple’ and other sites and apps that will rely on Apple’s SSO will also likely feature a similar login procedure.
In all likelihood, the company will also expand the use of such a login process to its other services and digital storefronts.