OS Details

Apple iPhone X has been launched in October 2017.
Apple iPhone X have iOS 11.1.1 , upgradable to iOS 11.2

Screen Dimensions

Apple iPhone X screen size is 5.8 inch with ~ 82.9% body ratio of actual device size. Physical dimensions for Apple iPhone X device is 5.65 x 2.79 x 0.30 INCH or ( 143.6 x 70.9 x 7.7 MM).

Displaying of Units are in this order "Height x Width x Thickness"

Screen Pixel Density and CSS Pixel Ratio

"Pixel" is smallest unit/element of any display.

The total number of pixels that fit into an inch is referred to as "Screen Density" or "Pixel Density" and it measured as "Pixels Per Inch".
Pixel Desnsity have limits to display Pixels Per Inch depends on different screen sizes. When number of pixels per inch increases than it's screen-size limit, the Display resolution will increase but actual device width/height in pixels remain same. Actual device pixels are known as Device Independent Pixels or CSS Pixel Ratio.

Apple iPhone X resolution is 1125 x 2436 ^PX with ~ 458 pixel desnsity.
Apple iPhone X viewport size is 375 x 812 ^PX with ~ 153 actual pixel density, which means it have 3 xxhdpi display pixel density.

Screen Resolution and Viewport

The sum of pixels which are displaying on a device is called as "Screen Resolution". and sum of actual pixels of any device is reffered to as "Viewport".

Apple iPhone X have 5.8" physical screen size and its resolution is about 1125 x 2436 Pixels with approximately 458 PPI pixel density. Apple iPhone X has viewport size 375 x 812 Pixels and its pixel ratio is about 3.

For better understanding screen actual sizes, viewport sizes, display resolution and about their density click here.

CSS Media Queries

Apple iPhone X Media Queries (In terms of Mobile only)

@media only screen and (min-width: 375px) and (max-width: 767px) { /* Your Styles... */ }

Apple iPhone X Min-Width Media Queries

@media only screen and (min-width: 375px) { /* Your Styles... */ }

Apple iPhone X Min-Height Media Queries

@media only screen and (min-height: 812px) { /* Your Styles... */ }

Apple iPhone X Landscape Media Queries

@media only screen and (min-width: 812px) and (orientation: landscape) { /* Your Styles... */ }

Apple iPhone X Portrait Media Queries

@media only screen and (min-width: 375px) and (orientation: portrait) { /* Your Styles... */ }

Apple iPhone X Retina Media Queries

@media
  only screen and (-webkit-min-device-pixel-ratio: 3),
  only screen and (   min--moz-device-pixel-ratio: 3),
  only screen and (     -o-min-device-pixel-ratio: 3/1),
  only screen and (        min-device-pixel-ratio: 3),
  only screen and (                min-resolution: 458dpi),
  only screen and (                min-resolution: 3dppx) { 
    /* Retina styles here */
}
                                

Retina is actually based upon device pixel ratio. Devices mostly have 2x or 3x display so you can use general retina media queries to display high resolution content on all kind of devices. Retina 2x and Retina 3x media queries given below:

Retina 2x Media Query

@media
  only screen and (-webkit-min-device-pixel-ratio: 2),
  only screen and (   min--moz-device-pixel-ratio: 2),
  only screen and (     -o-min-device-pixel-ratio: 2/1),
  only screen and (        min-device-pixel-ratio: 2),
  only screen and (                min-resolution: 192dpi),
  only screen and (                min-resolution: 2dppx) { 
    /* Retina styles here */
}
                                

Retina 3x Media Query

@media
  only screen and (-webkit-min-device-pixel-ratio: 3),
  only screen and (   min--moz-device-pixel-ratio: 3),
  only screen and (     -o-min-device-pixel-ratio: 3/1),
  only screen and (        min-device-pixel-ratio: 3),
  only screen and (                min-resolution: 384dpi),
  only screen and (                min-resolution: 3dppx) { 
    /* Retina styles here */
}
                                

Apple iPhone X Specifications

• Launch Date October 2017
• iOS 11.1.1
• Physical Screen Size 5.8 INCH
• Resolution 1125 x 2436 PX
• Viewport Size 375 x 812 PX
• Pixel Density 458 PPI
• Device-Independent Pixels 153 PPI
• Pixel Ratio 3 xxhdpi
• Width 1125 PX
• Height 2436 PX
• Device Width 375 PX
• Device Height 812 PX
• Dimensions 143.6 x 70.9 x 7.7 mm (5.65 x 2.79 x 0.30 in)
• Weight 174 g - 6.14 oz