definition

window.devicePixelRatio is the ratio between physical pixels and device-independent pixels (dips) on the device.
window.devicePixelRatio = physical pixels / dips

Dips are the abstract pixels that are used to feed information to the width/height media queries and the meta viewport device-width.

relation with other properties

dips:
设置页面的<meta name="viewport" content="width=device-width">，
获取document.documentElement.clientWidth，大部分浏览器会返回viewport layout的宽度，即为dips

summary

devicePixelRatio is mostly trustworthy on most browsers.
On iOS devices, multiply devicePixelRatio by screen.width to get the physical pixel count.
On Android and Windows Phone devices, divide screen.width by devicePixelRatio to get the dips count.