Code in iOS 11.3 beta hints at a single sign-in system for websites using Apple ID
Another feature will allow a person with an iPhone or iPad to scan a QR code with his camera, and receive a request to sign-in to a website using his Apple ID. Allowing iOS users to log into websites in this manner, will make it easier for them to access third party sites; it is much easier to remember one Apple ID as opposed to a larger number of usernames and passwords.
There is no guarantee that this system will make it to the final version of iOS 11.3, although there are at least a couple of reasons why it should. One, Apple already has a single sign-in system for Apple TV. Two, this would be too useful to iOS users for Apple to eliminate.