StackOverflow user ilyo was trying to figure out what exactly is Device Pixel Ratio, and asked for some elaboration.

Short answer

The device pixel ratio is the ratio between physical pixels and logical pixels. For instance, the iPhone 4 and iPhone 4S report a device pixel ratio of 2, because the physical linear resolution is double the logical linear resolution.

• Physical resolution: 960 x 640
• Logical resolution: 480 x 320

Other devices report different device pixel ratios, including non-integer ones. For example, the Nokia Lumia 1020 reports 1.6667, the Samsung Galaxy S4 reports 3, and the Apple iPhone 6 Plus reports 2.46 (source: dpilove)). But this does not change anything in principle, as you should never design for any one specific device.

Discussion

The CSS "pixel" is not even defined as "one picture element on some screen", but rather as a non-linear angular measurement of 0.0213° viewing angle, which is approximately 1/96 of an inch at arm's length. Source: CSS Absolute Lengths

This has lots of implications when it comes to web design, such as preparing high-definition image resources and carefully applying different images at different device pixel ratios. You wouldn't want to force a low-end device to download a very high resolution image, only to downscale it locally. You also don't want high-end devices to upscale low resolution images for a blurry user experience.

If you are stuck with bitmap images, to accommodate for many different device pixel ratios, you should use CSS Media Queries or the HTML picture element to provide different sets of resources for different groups of devices. Combine this with nice tricks like background-size: cover or explicitly set the object-fit property to control the image rendering.

Example

The CSS below will let each device type only load the correct image resource. Also keep in mind that the px unit in CSS always operates on logical pixels.

#element { background-image: url('lores.png'); }

@media only screen and (min-device-pixel-ratio: 2) {
    #element { background-image: url('hires.png'); }
}

@media only screen and (min-device-pixel-ratio: 3) {
    #element { background-image: url('superhires.png'); }
}

A case for vector graphics

As more and more device types appear, it gets trickier to provide all of them with adequate bitmap resources. In CSS, media queries is currently the only way, and in HTML5, the picture element lets you use different sources for different media queries.

If you need crisp images for icons, line-art, design elements that are not photos, you need to start thinking about SVG, which scales beautifully to all resolutions.