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Asked: 2026-05-16T14:01:58+00:00

Mozilla’s documentation from elementFromPoint explains that the coordinates are not in physical pixels, but

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Mozilla’s documentation from elementFromPoint explains that the coordinates are not in physical pixels, but “CSS pixels”. Exactly what are CSS pixels? I was under the impression that a pixel in CSS was the same as the physical pixel.

If this is not the case, how does one convert between physical and CSS pixels?

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1 Answer

  1. Editorial Team

    A pixel is a physical screen pixel as far as any web page can reasonably concern itself.

    The wording ‘CSS pixel’ refers to a pixel as defined in CSS 2

    the whole number of device pixels that best approximates the reference pixel. It is recommended that the reference pixel be the visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm’s length. For a nominal arm’s length of 28 inches, the visual angle is therefore about 0.0213 degrees.

    What this is saying is that a CSS pixel is a device pixel for the normal, simple screen cases. However, for specialist super-high-res screens where the OS scales up its normal dimensions by two, a CSS pixel may be two device pixels wide.

    We now also have a ‘zoom’ feature in most browsers, which will naturally change the size of the CSS pixel (along with all other units) so it doesn’t match a device pixel.

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