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Re: size of text when determining color contrast

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From: Patrick H. Lauke
Date: Oct 6, 2014 11:01AM

On 06/10/2014 17:15, Whitney Quesenbery wrote:
> Point sizes area a handy shorthand for measurements of physical size,
> because they are a fraction of an inch. .5 inches (or 36 points) is .5
> inches, no matter what screen the text is on.

Actually, that's not quite correct. In the old definition for CSS
values, this was always true...but no device actually implemented this
part. Instead - with the exception of print, if I'm not mistaken - all
measures like inches, millimeters, points etc are anchored to the
*reference pixel*, which varies across devices and is itself related to
the average viewing distance of a screen.

http://www.w3.org/TR/css3-values/#absolute-lengths

This is actually one of my bugbears with WCAG 2.0 ... talking about
measurements in points is irrelevant for the most part, as there is no
reliable way to relate "pt" to the real physical size of text. Viewing
distance is also a factor...in theory, if devices do correctly set their
viewport so that a CSS pixel matches the ideal reference pixel closely,
good times...a "pt" will then be close to the idealised "pt" taking into
account viewing distance. However, even this relationship is broken in
the case where browsers don't have a viewport matching the ideal
viewport (for instance, a site viewed on a mobile device, but lacking
the meta viewport width=device-width, meaning that the browser will use
some default width like 980px or similar, which then makes a CSS pixel
far smaller than the reference pixel, and ergo a "pt" is much smaller
than what it should ideally be).

Anyway, long story short, beware that "pt" or similar measurements can
be completely divorced from reality (and, from that perspective, are
fairly useless as a measurement for WCAG 2.0 purposes).

P
--
Patrick H. Lauke

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