Device Pixel Ratio (DPR)

The Device Pixel Ratio indicates the density of pixels on a device's screen. High-resolution displays (like Apple's "Retina" screens) have a DPR of 2 or more, meaning they use a 2x2 grid of physical pixels to render what is a single CSS pixel. This is important for Image Optimization, as you should serve higher-resolution images to these devices for sharpness, but not images that are so large they waste bandwidth and slow down loading.