These are both correct. The CSS 2.1 spec actually defines a CSS pixel ("px") as ... | Hacker News

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jahewson on Nov 4, 2013 | parent | context | favorite | on: In CSS, “px” is not an angular measurement and it ...

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 on Nov 5, 2013 [–]

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 on Nov 5, 2013 | [–]

Right, so a CSS pixel is equal to the reference pixel in size.

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