By default, the new iPhone X in landscape mode will contain sites in the so called “safe area”, resulting in white bars being rendered on either side of the site(src).
The color, white by default, can be tweaked by altering the background-color on the <body> element. Do note that it’s only background-color though: it doesn’t take gradients/background images into account, so you won’t jump very far with this …
Cover it up!
By adding viewport-fit=cover to the viewport meta tag, it’s possible to make the site stretch out beyond the safe area so that it takes up the full width of the device (src)