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Re: How were the WCAG contrast ratios determined

for

From: Patrick H. Lauke
Date: Feb 12, 2016 7:14AM

On 12/02/2016 14:02, Tim Harshbarger wrote:
> The "Understanding" page for success criteria 1.4.3 (Contrast [Minimum]) provides rationale for the numbers:
> http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
>
>
> I just reread the entry. To me, it seems like the ratios would stay the same despite changes in luminance of monitors. What might change is how the ratio is calculated.

From my reading, no particular display technology was actually
considered for the ratios, but rather on some form of idealised model of
color rendition.

Though I don't think anybody's suggestion this, I'd be careful if this
line of reasoning moves towards the idea that these ratios should be
measured "on the screen", so to speak. There are many factors - type of
display, color correction profiles, user-set contrast/brightness,
ambient light condition, etc - which are outside of the control of
developers, are extremely variable across even the same device
type/model, and cannot (reliably, or at all) be programmatically
detected. Even just for auditing/testing, this would very quickly result
in the same color values returning different ratios (and potentially
differences between a pass and a fail) simply based on the auditor's
current screen/setup.

As such (as with the note on large text, which has exactly the same
issues of variability in the real world) the only reliable, measurable
and stable reference to calculate ratios has to be the values "obtained
from the user agent" - and then assume the device/display render those
values in whatever they deem is the most faithful/appropriate way (and
knowing that users may still change this themselves anyway).

P
--
Patrick H. Lauke

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