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These are both correct. The CSS 2.1 spec actually defines a CSS pixel ("px") as "the whole number of device pixels that best approximates the reference pixel".

These are both correct. The CSS 2.1 spec actually defines a CSS pixel ("px") as "the whole number of device pixels that best approximates the reference pixel". It's to allow a little leeway for devices with low-resolution screens. On a high-resolution screen a CSS pixel should be = the reference pixel.

Macha 13y ago
To the best of my knowledge, on iOS, 1 CSS px has been 2 device pixels for a while. Might even be 4 device pixels on Retina devices.
- jahewson 13y ago
  Right, so a CSS pixel is equal to the reference pixel in size.