How w3.org authors translate a visual angle in height to measure the dimension of a CSS pixel's reference?

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Im wondering where they pull the number 0.26mm, here the link of the documentation: https://www.w3.org/TR/css-values-4/#absolute-lengths

Here the authors statement:

The reference pixel is 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. For reading at arm’s length, 1px thus corresponds to about 0.26 mm (1/96 inch).

I would understand the calculus behind this result. How they have fuond 0.0213degrees? how they have found that 1px thus corresponds to about 0.26 mm? that is my very question please

any hint would be great,

thanks

math css
1 Answers

With an arm length of 28 inches, the circumference of the circle is 28 * 2pi = 175.929 inches. We can then take the size of a pixel (1/96 inch, per the spec) and divide it by the circumference to determine the proportion of the circle that is a single pixel. ([1/96]/175.929). Multiplying this value by 360 converts the proportion to degrees. The final calculation is (1/96)/(28*2pi)*360 = 0.0213°.

As to the second part of your question, they didn't find that 1px is 1/96 of an inch — that's actually how an inch is defined in CSS. If you create a line with length 1in, it will always be 96px, regardless of the pixel density of your screen.

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