Here you go:

> Apple App Review Guidelines: 2.5.1 Apps may only use public APIs and must run on the currently shipping OS.

https://developer.apple.com/app-store/review/guidelines/

And here's the (private) field that needs to be enabled on your webview to enable the CSS property. Otherwise (according to the author) it's just ignored: https://github.com/WebKit/WebKit/blob/613c42873c56e2b2073f91...