iPhone X

@media only screen 
    and (device-width : 375px) 
    and (device-height : 812px) 
    and (-webkit-device-pixel-ratio : 3) { }

iPhone 8

@media only screen 
    and (device-width : 375px) 
    and (device-height : 667px) 
    and (-webkit-device-pixel-ratio : 2) { }

@media only screen 
    and (device-width : 414px) 
    and (device-height : 736px) 
    and (-webkit-device-pixel-ratio : 3) { }

iPhone 6+/6s+/7+/8+ share the same sizes, while the iPhone 7/8 also do.

Looking for a specific orientation ?

Portrait

Add the following rule:

    and (orientation : portrait)

Landscape

Add the following rule:

    and (orientation : landscape)

References: