Bit of an idle question as I'm not using pixels for anything except borders, but I'm trying to understand as much as I can about this.

I've read that high pixel density devices - at least usually - compensate for CSS declarations in a stylesheet that should only work in devices where a device pixel = a CSS pixel.

e.g. a CSS style says 'width=100px' and a Retina device will render that at 200px. This is why sites don't look half their normal size on Retina devices.

But... what if you write a media query in pixels. What sort of pixels are used?
- dppx
- dpi / dpcm

Does it depend on the browser support for the pixels i.e. devices that support dppx use that and devices that don't use dpi (CSS pixels)?