Apple has updated the support page, to confirm that any third-party USB-C power adapter that supports USB Power Delivery (USB-PD) can be used to fast charge the iPhone 8/X. Enjoy
https://support.apple.com/en-us/HT208137
Edit: An official USB-C to Lightning cable is of course needed (third-party cable will not work)
https://support.apple.com/en-us/HT208137
Edit: An official USB-C to Lightning cable is of course needed (third-party cable will not work)