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Re: colour contrast algorithm

From: Joe Clark <joeclark@joeclark.org>
Date: Fri, 19 Sep 2003 11:27:39 -0400 (EDT)
To: WAI-IG <w3c-wai-ig@w3.org>
Message-ID: <Pine.BSO.4.53.0309181804120.28515@mail.veldt.ca>

> The W3C's draft techniques document for Accessibility Evaluation and
> Repair Tools at [15]http://www.w3.org/TR/AERT#color-contrast provides an
> algorithm for testing to see if text and background colours provide
> sufficient contrast.

That issue cannot be programmatically determined, full stop. Every
visually-impaired person is different. It's *not* like colour deficiency,
which has well-understood patterns and easy-to-follow accommodations.

Cf.:

<http://joeclark.org/f2f/deficiencies.html#colour-structure-impl>
<http://joeclark.org/f2f/recommendations.html#colour>

> The document indicates, however, that the algorithm is a "suggested" one
> that is "still open to change".  Can anyone shed any light on whether
> this should be considered as a reliable indicator of colour contrast, or
> whether some more authoritative/reliable algorithm exists?

No and no.

> Reason for my question: sponsors/clients of a project I am currently
> working on believe that the requirements are too stringent. This view
> was expressed after I used the algorithm to generate a report on a range
> of colour options that were being considered--very few of them passed
> muster, as I expected.

The correct accommodation method is for an author to provide a means of
altering colours, as with a stylesheet-switcher.

The visitor can also adjust browser controls or employ a user stylesheet
including !important declarations.

--

  Joe Clark  |  joeclark@joeclark.org
  Author, _Building Accessible Websites_
  <http://joeclark.org/access/> | <http://joeclark.org/book/>
Received on Friday, 19 September 2003 11:26:20 GMT

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