[css-values] clarify that most (all?) high-dpi devices anchor on pixel, rather than physical, unit

[patrickhlauke]

The section on absolute lengths https://drafts.csswg.org/css-values/#absolute-lengths currently contains the following:

For print media and similar high-resolution devices, the anchor unit should be one of the standard physical units (inches, centimeters, etc). For lower-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.

In essence, this splits devices into "print and high resolution" on one side, and "lower-resolution and unusual viewing distances". However, most (all?) high-dpi devices (including recent desktop monitors, laptops, mobile/tablet devices) currently also anchor on the pixel unit. It seems that it's really only print media that (try to) anchor on physical units (unless I'm missing some non-print device that does this?).

I think it would be important to either rewrite this section to change the distinction, or to leave the above as is but then include a note straight after clarifying that currently even high-resolution desktops/laptops/mobiles/tablets are anchored on the pixel unit.

The reason for bringing this up is that this issue seems to constantly rear its head in discussions around being able to use absolute measures like "mm" to actually map to physical size of elements as rendered on a device (most recently I had this discussion in the mobile accessibility task force https://www.w3.org/WAI/GL/mobile-a11y-tf/wiki/Summary_of_Research_on_Touch/Pointer_Target_Size#Comparison_of_web_content_sized_in_px_and_physical_size_of_rendered_content_on_common_device_screens which led to having to demonstrate this practically using http://codepen.io/patrickhlauke/full/zqabMR/ ... and I'm aware of similar discussions under WICG in relation to large screen / TV environments)

Happy to draft a PR if this sounds like a reasonable request.

[nicksherman]

Related (perhaps duplicate?): Issue #614

[patrickhlauke]

related yes. not duplicate though, I'd say (as #614 is proposing a change in what UAs do / new units of measure, while here I'm proposing making the spec match/describe reality as it stands a bit more closely)

[frivoal]

I agree with this. This isn't really changing how things work, a the way units are anchored to eachother stays the same, and the recommendation to anchor the whole system to either the physical units is only a recommendation, not a requirement. But it is indeed a recommendation that does not match reality, and I don't think it is particularly good advice: what browsers do today is the right thing to do.

[patrickhlauke]

@frivoal cool i'll draft a PR in the coming days and fling it over

[fantasai]

Wrt the PR here (which, btw, fails to edit css-values-3), it seems to categorize print media with unusual viewing distances into the physical-anchor category, whereas before they were grouped with the pixel-anchor category. If this was an intentional change, I would like a clear resolution on that change from the WG. If that is not an intentional change, then I intend to revert it.

@patrickhlauke can you clarify your intent?

[patrickhlauke]

Wrt the PR here (which, btw, fails to edit css-values-3),

oh, does that mean this PR targetted the wrong branch or file? should i resubmit the PR targetting the right one (which one?)

it seems to categorize print media with unusual viewing distances into the physical-anchor category, whereas before they were grouped with the pixel-anchor category.

@fantasai as the second part of that section even in the original didn't explicitly say "print", it didn't occur to me here that it even covered "print media with unusual viewing distances" (and i struggle to think of such a device to be honest). But thinking about it now I'd say that once it's print, shouldn't it still be anchored to physical size regardless of what viewing distance the end result is then intended to be viewed at (since the author would presumably account for this, and do so by specifying sizes again with an expectation of them being anchored on actual physical sizes). Also, to make a simplistic analogy, how would a UA/printer know if something's intended to be viewed at a "usual" or "unusual" distance?

in short, it was not my intention of redefining the anchoring of "print media with unusual viewing distances" when I made the PR, as it was not clear to me that this text implicitly covered it. but i'd say print should always be anchored on physical units regardless