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Re: [csswg-drafts] [css-values] Ability to address actual physical size

From: Florian Rivoal via GitHub <sysbot+gh@w3.org>
Date: Wed, 19 Oct 2016 00:44:28 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-254679777-1476837867-sysbot+gh@w3.org>
> > the current definitions are not merely an accident of history, 
they are intentional.
> The current definitions were not the original intention of the spec.


> The angular measurement ascribed to px in the spec (0.0213 degrees) 
is fairly arbitrary, a backwards-justification from the previous 
definition of px in order to maintain some kind of logic and support 
older designs. If the original plan was to include angular 
measurements in the spec, they would have chosen a more logical unit 
like degrees or arcminutes.

Ok, sure, there is of course some part of the current definition that 
can be explained by history. The fact that we use an angular 
measurement called pixel rather than degrees or arcminutes is 
certainly due to the fact that we started with a pixel unit. You are 
right, it initially did not have that behavior, and was simply a 
physical pixel. But as time went by, that definition was found to be 
problematic, and the newer system was used to redefine it.

What is particularly useful about the angular definition of the pixel 
and other length units is that they enable robust designs. By that I 
mean that it enables authors to write a web page that works in 
environments they know about, **and be confident that it will do the 
right thing even in environments they haven't tested in or are not 
even aware of**. 

For instance, when then first iphone came out, it had small physical 
pixels, but since they were to be viewed from a close distance, it was
 still OK to have 1px to be 1 physical pixel. That meant that inches 
on the iphone were small, since they kept the 96 to 1 ratio. This in 
turn meant that sites that had not anticipated the iphone ended up 
working just fine. Further down, when retina iphones came out, pixels 
were now “too” small, so we got multiple device pixel per css pixel, 
inches stayed the same size, and again everything worked fine. And not
 everybody thinks of a projector or a nintendo wii when they design 
their site, but again, it just works.

I am not claiming that there is no possible use for physical 
measurements. Authors are infinitely creative, so I am sure something 
could be made of them. On the other hand, I *am* sure that valid uses 
of physical measurements are many orders of magnitude more rare than 
the ones we have now.

This gives a double challenge to people asking for physical 

1. Make sure you design this ability to access physical measurements 
in such a way that it could not possibly confuse authors, and cause 
even a small fraction of them to use physical measurements when they 
should be using the system we have now. Otherwise, such confused 
authors will write sites that don't work well across devices, and make
 the overall web experience worse for end users. If one author in a 
million makes the mistake, it is probably fine, but if one in a 
hundred does and writes sites that don't adapt well to different 
environments due to this, we will have done more harm than good.

2. Since the use cases, even if real, are rare, convince web browser 
vendors that this is worth their while. They already have a long list 
of things they mean to get to, and currently none of them recognize 
this need as something relevant, this is going to be an uphill battle.

With regards to (1), I would say that introducing new units that give 
access to physical distances would not be acceptable. There's just too
 much a chance that authors who haven't thought deeply about this 
would pick the physical ones instead of the angle based ones, and make
 brittle designs because of that. So if this is to be exposed, it has 
to be some other way.

As for (2), I suggest trying to build a corpus of concrete examples. 
Not abstract declarations of reasons why it should be useful, actual 
specific examples of designs that you cannot do today, but could if 
you had the feature. Down to the actual code, with mock ups of the 
rendering (ascii art will do, but the point is to be concrete), 
reasons why you cannot achieve this without the feature you're asking 
for, and (if it is not painfuly obvious) reasons why there is merit to
 this design.

>  If nothing else, the ability to address physical size is 
fundamental to using CSS for any fixed media like print. If designers 
can’t render an element at a specific physical size on a piece of 
paper, the whole notion of CSS being a truly universal language for 
presenting a document in a variety of media (including print) is moot.

>From the spec:

> For print media [...] the anchor unit should be one of the standard 
physical units (inches, centimeters, etc)

So that's solved already. Using CSS for print is not theoretical, and 
I am not just talking about pressing the print button in your browser.
 Commercial books are made with CSS all the time (I just typeset [this

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Received on Wednesday, 19 October 2016 00:44:37 UTC

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