Apple's iOS 11.3 may use iCloud as a single sign-on for websites
 

You might not to log in to your favorite websites one at a time in the near future. The 9to5Mac team has found code in the iOS 11.3 beta hinting at a single sign-on option for the web based on your iCloud account. It's not certain just how it would...

https://www.engadget.com/2018/02/03/...-for-websites/