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Re: size of text when determining color contrast

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From: Jukka K. Korpela
Date: Oct 6, 2014 11:28AM

2014-10-06 20:01, Patrick H. Lauke wrote:

> 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.

Well, that's at least what we can read between the lines in CSS3 specs.
But even in print, the in unit does not usually match the physical unit
of inch exactly. Anyway, what seems to be more important is that WCAG
2.0 defines "large text" in a rather complicated and vague way
(inaccessibly, I dare say):
http://www.w3.org/TR/WCAG20/#larger-scaledef
It says: "with at least 18 point or 14 point bold or font size that
would yield equivalent size for Chinese, Japanese and Korean (CJK) fonts".

> 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.

I think that despite all the vagueness and obscurity, the WCAG 2.0
definition is still useful. In modern CSS specs, 1pt equals 4/3 "pixels":
http://www.w3.org/TR/css3-values/#absolute-lengths

It's the "pixel" (px) concept that varies by device. Well, at least to
the same extent as any other CSS "absolute" length unit.

So what is the practical problem? WCAG 2.0 gives criteria for "large"
text. The physical meaning of pt valued (and other) font sizes varies,
but conformance to WCAG 2.0 is still achieved if you meet the
requirements using *its* definitions for "large text".

Yucca