Re: [csswg-drafts] [css-fonts] Proposal to extend CSS font-optical-sizing (#4430)

It’s worth noting that the nature of a CSS `pt` (as well as `px`, `in`, `cm`, and all the other compatible “absolute” CSS units) actually changes between a fixed/physical measure and a relative/angular measure depending on the context (as if everything wasn’t already confusing enough). 

To quote the W3C’s explanation:
> For print media at typical viewing distances, the anchor unit should be one of the standard physical units (inches, centimeters, etc). For screen media (including high-resolution devices), low-resolution devices, and devices with unusual viewing distances, it is recommended instead that the anchor unit be the pixel unit. For such devices it is recommended that the pixel unit refer to the whole number of device pixels that best approximates the reference pixel.

So, for print, 1 CSS point should **theoretically** be the same as 1 traditional, physical point. For screen-based media, 1 CSS point should **theoretically** be about 0.0284 degrees when viewed from a typical distance. (As I mentioned before, though, the practical realities are very different. Even in print, the absolute CSS units are rarely reliable.)

The point of bringing this up is to show that mapping `opsz` to CSS points isn’t exactly the same thing as mapping to an angular measure, and definitely not one that is straightforward, reliable, intuitive, and easy to measure or test against.

If the OpenType spec adapted a more straightforward angular unit for the definition of `opsz`, user agents could interpret that value however was most appropriate for them, referencing whatever information they have (or don’t have). The results may vary from one place to another, but at least the design for the typeface can be very intentional and precise, based on a reliable and easily testable optical size. Otherwise, maybe don’t bother calling it “optical size”.

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Received on Wednesday, 3 June 2020 01:40:25 UTC