Apple have posted this to give you some guidance on page design for the iPhone X, particularly where the device’s sensor housing takes up part or the view and will obscure part of the page display.
To help with this they have introduced some new WebKit APIs, Full details in the link below.
https://webkit.org/blog/7929/designing-websites-for-iphone-x/
also check out some other new features in webkit
https://webkit.org/blog/7956/new-webkit-features-in-safari-11/