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Re: Color Contrast and Transparency | WCAG 2.1

From: Alastair Campbell <acampbell@nomensa.com>
Date: Fri, 29 Jul 2016 08:39:44 +0000
To: WCAG <w3c-wai-gl@w3.org>, W3C WAI Accessible Platform Architectures <public-apa@w3.org>
Message-ID: <429C7729-A952-41E1-BC16-07EF98A492DD@nomensa.com>
JF: “the impact of Alpha Transparency of colors and the impact on color contrast and accessibility?”

I guess this is mostly a matter of how it is measured?

So if designer/developer specifies a foreground and/or background colour with some transparency, you shouldn’t use the CSS defined colour to test contrast without accounting for the transparency. (I know transparency is generally defined with the colour, but that doesn’t mean a testing tool accounts for it.)
If the transparent colour of a box (the background for text) is on top of a white background it will be lighter than on a black background, which needs to be accounted for.

However, if you use a tool with an eye-dropper style mechanism on the resulting colours, that should be accurate. Unless I’m missing something?

The contrast adjuster function in the CSS-color-4 spec looks cool (if it’s implemented?), it references WCAG2 1.4.3 so should line up. 

Perhaps we need to make G18 more specific about testing the result rather than the colour definition?

And perhaps we could create a test page that would pass if transparency is not accounted for, but should fail, and check that each of the tools listed in G18 accounts for the transparency? (Perhaps the browser based ones don’t?)

Cheers,

-Alastair

Received on Friday, 29 July 2016 08:40:20 UTC

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