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Re: notes on 320 CSS Pixels to inches

From: Patrick H. Lauke <redux@splintered.co.uk>
Date: Thu, 18 Jan 2018 18:22:54 +0000
To: w3c-wai-gl@w3.org
Message-ID: <8ce2187f-61c8-9b4c-f281-7018c8bc6244@splintered.co.uk>
On 18/01/2018 14:44, Hakkinen, Mark T wrote:
> And this specific discussion is why I have such a hard time understanding why we don't define the user's perceived physical, on screen size of the button as the minimum in the SC.  The majority of the research (and I've spent the past few days looking at over 500 pages of research studies on touch and target size) describe the stimulus used as the target in millimeters (aids replicability of research).  The human engineering design guidelines used in aerospace or the accessibility design guidelines for elevator controls define button size as a physical measure.  These are precise and meet the needs of the user.  When we talk about CSS Pixels, and define minimum target size in terms of pixels in a world of many display sizes and true pixel densities, there appears to be a broad acceptance that, yes, there is a fudge factor... the true measured size will vary, and that's okay.  I tested a number of devices and saw that variability.    Does that serve the user?   That the concept of CSS Pixels is hard to describe (as has been noted in many places), makes this SC problematic without also stating clearly the physical, measure on-screen physical minimum in an international unit of measure (mm).

Authors can't define how big anything they define is going to be 
rendered at. Any measurement you define when making web content in your 
CSS (be it px, em, mm, cm, pt) is anchored on the idealised CSS pixel. 
If I, as an author, define some measurement in, say, mm

button { width: 10mm; height: 10mm; }

browsers will simply convert that measurement down into a set number of 
pixels in all browsers, namely 37.7813px (as an exercise to the reader, 
you can work that out by following the various relationships/conversions 
in https://www.w3.org/TR/CSS21/syndata.html#length-units)

This happens regardless of the size of the monitor. Meaning that there 
is zero guarantee that "10mm" defined in CSS will actually result in 
10mm as measured on the actual physical screen.

Author have zero control over achieving an SC that mandates a physical 
(as measured in real world on the screen) size. And funnily enough, 
auditors, when testing if content passes or fails, will also not have 
any guarantee that their test is accurate, as they will get different 
results depending on which particular device, mobile phone, etc they 
test on.

WCAG already made this fundamental mistake (of basing something on 
real-world measurements, since they borrowed it from the print world) 
when defining "large scale text" for contrast. Please, let's not make 
this same mistake again...

Patrick H. Lauke

www.splintered.co.uk | https://github.com/patrickhlauke

http://flickr.com/photos/redux/ | http://redux.deviantart.com

twitter: @patrick_h_lauke | skype: patrick_h_lauke
Received on Thursday, 18 January 2018 18:23:18 UTC

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