How do high-DPI devices like Retina displays and 1080p phones handle pixels in media queries?

Question

For example, how many pixels does the iPhone with a Retina display appear to a media query? Is it 320 pixels or 640 pixels? If it's 640 pixels, how can a website deal with different device resolutions? Is there any way to use units that are the same regardless of a screen's resolution?

Answer 1

Retina devices like the iPhone and iPad usually use 2 device pixels = 1 CSS pixel. So a media query like this:

@media all and (max-width: 320px) {
}

actually will target an iPhone (portrait), even though it is 640 device pixels wide.