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Re: F24 - contrast

From: james nurthen <james.nurthen@oracle.com>
Date: Wed, 18 Dec 2013 11:04:55 -0800
Message-ID: <52B1F1D7.6020509@oracle.com>
To: w3c-wai-gl@w3.org
I'm not sure we can change this as "contrast ratio" is defined in WCAG 
including Note 4 where this is specifically called out as a failure.


contrast ratio

    (L1 + 0.05) / (L2 + 0.05), where


        L1 is therelative luminance
        the lighter of the colors, and


        L2 is therelative luminance
        the darker of the colors.

    /Note 1:/Contrast ratios can range from 1 to 21 (commonly written
    1:1 to 21:1).

    /Note 2:/Because authors do not have control over user settings as
    to how text is rendered (for example font smoothing or
    anti-aliasing), the contrast ratio for text can be evaluated with
    anti-aliasing turned off.

    /Note 3:/For the purpose of Success Criteria 1.4.3 and 1.4.6,
    contrast is measured with respect to the specified background over
    which the text is rendered in normal usage. If no background color
    is specified, then white is assumed.

    /Note 4:/Background color is the specified color of content over
    which the text is to be rendered in normal usage. It is a failure if
    no background color is specified when the text color is specified,
    because the user's default background color is unknown and cannot be
    evaluated for sufficient contrast. For the same reason, it is a
    failure if no text color is specified when a background color is

    /Note 5:/When there is a border around the letter, the border can
    add contrast and would be used in calculating the contrast between
    the letter and its background. A narrow border around the letter
    would be used as the letter. A wide border around the letter that
    fills in the inner details of the letters acts as a halo and would
    be considered background.

    /Note 6:/WCAG conformance should be evaluated for color pairs
    specified in the content that an author would expect to appear
    adjacent in typical presentation. Authors need not consider unusual
    presentations, such as color changes made by the user agent, except
    where caused by authors' code.

On 12/12/2013 6:52 AM, Adam Solomon wrote:
> Firefox and ie (and perhaps others) allow the user to override css 
> colors AND bg images - so does that mean that to fulfill the criterion 
> an author would simply have to use a technology which works in modern 
> browsers? (even if the css colors failed the contrast ratio) - 
> essentially saying that we don't have to check contrast on web pages 
> anymore
> On Wed, Dec 11, 2013 at 8:39 PM, Ramón Corominas 
> <rcorominas@technosite.es <mailto:rcorominas@technosite.es>> wrote:
>     Hi, David and all,
>     Windows high contrast mode changes any foreground colour to the
>     selected "automatic" text and any background colour to the
>     selected automatic background colour (remember that Windows has
>     different high contrast modes, not all of them use dark backgrounds).
>     In any case, links are not changed to the automatic foreground
>     colour, but to the default link text, which sometimes can provoke
>     difficulties. Nevertheless, most users of high contrast that I
>     know also change this link colour to meet their needs, either in
>     the operating system or in the browser itself (browsers usually
>     have an option to override system colours).
>     However, in technologies different from HTML, colours are not
>     managed the same, and for example Word 2010 changes foreground
>     colours to "automatic", but leave background colours as defined by
>     the author, which in many cases provoke "white over light gray";
>     in addition, I think Adobe Reader does not change any colour
>     unless the reflow option is used.
>     So, if the Failure only applies to HTML, then it should probably
>     be removed, but if it applies to any technology, it is still valid.
>     Regards,
>     Ramón.
>     David wrote:
>     > The history of this is to enable users to *switch* colours (black to
>     > white and vice versa) without the hard coded colours preventing then
>     > change... so it black switches to white in the background but the
>     > foreground doesn’t switch then you have black text on black
>     > background... this is one of those success criteria that we will
>     want to
>     > look closely at, because as far as I can tell AT that switches
>     colours
>     > such as zoomtext will override even hard coded colours and
>     successfully
>     > make the change, We would have to check Windows high contrast
>     mode, but
>     > I think it does the same thing... so I question whether we should
>     > continue to fail it...
>     >
>     >
>     >>
>     >> is F24 - http://www.w3.org/TR/2013/NOTE-WCAG20-TECHS-20130905/F24
>     >>
>     >> a failure even if the actual contrast meets the minimum contrast
>     >> requirement ratio? In other words, if an author were to specify
>     black
>     >> text color in css (and left the default background-color of say
>     white)
>     >> where the ratio meets the success criterion de facto, would
>     this still
>     >> fail since no bgcolor was specified in the css (because the
>     user could
>     >> run into problems if he did choose a different default
>     background color
>     >> in his browser settings)?
Received on Wednesday, 18 December 2013 19:05:27 UTC

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