OS Details

Xiaomi Mi 9 has been launched in March 2019.
Xiaomi Mi 9 have Android 9.0 (Pie) , upgradable to Android 10, MIUI 12

Screen Dimensions

Xiaomi Mi 9 screen size is 6.39 inch with ~ 85.2% body ratio of actual device size. Physical dimensions for Xiaomi Mi 9 device is 6.20 x 2.94 x 0.30 INCH or ( 157.5 x 74.7 x 7.6 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.

Xiaomi Mi 9 resolution is 1080 x 2340 ^PX with ~ 403 pixel desnsity.
Xiaomi Mi 9 viewport size is 393 x 851 ^PX with ~ 147 actual pixel density, which means it have 2.75 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".

Xiaomi Mi 9 have 6.39" physical screen size and its resolution is about 1080 x 2340 Pixels with approximately 403 PPI pixel density. Xiaomi Mi 9 has viewport size 393 x 851 Pixels and its pixel ratio is about 2.75.

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

CSS Media Queries

Xiaomi Mi 9 css media queries will help developers to write styles specifically for Xiaomi Mi 9. Here are details:

Xiaomi Mi 9 Media Queries (In terms of Mobile only)

@media only screen and (min-width: 393px) and (max-width: 767px) { /* Your Styles... */ }

Xiaomi Mi 9 Min-Width Media Queries

@media only screen and (min-width: 393px) { /* Your Styles... */ }

Xiaomi Mi 9 Min-Height Media Queries

@media only screen and (min-height: 851px) { /* Your Styles... */ }

Xiaomi Mi 9 Landscape Media Queries

@media only screen and (min-width: 851px) and (orientation: landscape) { /* Your Styles... */ }

Xiaomi Mi 9 Portrait Media Queries

@media only screen and (min-width: 393px) and (orientation: portrait) { /* Your Styles... */ }

Xiaomi Mi 9 Device Width Media Queries

@media only screen and (min-device-width: 393px) and (orientation: portrait) { /* Your Styles... */ }
@media only screen and (min-device-width: 851px) and (orientation: landscape) { /* Your Styles... */ }

Xiaomi Mi 9 Retina Media Queries

@media
  only screen and (-webkit-min-device-pixel-ratio: 2.75),
  only screen and (   min--moz-device-pixel-ratio: 2.75),
  only screen and (     -o-min-device-pixel-ratio: 2.75/1),
  only screen and (        min-device-pixel-ratio: 2.75),
  only screen and (                min-resolution: 403dpi),
  only screen and (                min-resolution: 2.75dppx) { 
    /* 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 (for all 2x resolution supported devices)

@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 (for all 3x resolution supported devices)

@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 */
}
                                

Xiaomi Mi 9 Specifications

• Launch Date March 2019
• Android 9.0
• Physical Screen Size 6.39 INCH
• Resolution 1080 x 2340 PX
• Viewport Size 393 x 851 PX
• Pixel Density 403 PPI
• Device-Independent Pixels 147 PPI
• Pixel Ratio 2.75 xxhdpi
• Width 1080 PX
• Height 2340 PX
• Device Width 393 PX
• Device Height 851 PX
• Dimensions 157.5 x 74.7 x 7.6 mm (6.20 x 2.94 x 0.30 in)
• Weight 173 g - 6.10 oz