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

Then if that’s the best it can be done, that an actual inch is 96 device pixels, I agree.

On May 27, 2020, at 4:01 PM, John Hudson <> wrote:

Rasterisation is a whole other topic. I'm talking about glyph outline design for specific optical sizes, which has to target an ideal physical size. Targeting outlines to specific ppem sizes is a different task—what we used to do with hinting. For this discussion, I'd like to leave rasterisation out of it, not least because the pixels involved in ppem raster sizes are real pixels, not CSS pixels.

In this thread, it is clearly stated that CSS px 'are not defined to have any physical length', and that relationship of a CSS inch to a physical inch is, hence, variable. Unless you can demonstrate that this is not the case—that CSS pixels are an absolute physical measurement—there is no point proposing them as a unit for optical size design, because it isn't possible to make size-specific design adjustments if they may be display up to ±33% different size in different places. That's the difference between 9pt and 12pt, which is precisely the range of optical size where very significant design variation occurs.

It seems to me, the best one could do in redefining the opsz scale to use 'px' as a unit would be to apply a special definition of that unit as being 1/96 of a physical inch, which I suppose would address what some browsers are doing now while still providing type designers with an absolute size target. But that would have to be very clearly stated in the spec, and would mean that some environments should really be differentiating their internal CSS px sizes from the px size used in the opsz scale if the former were different from a physical inch.

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Received on Wednesday, 27 May 2020 20:16:19 UTC