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A few issues with the proposed definition of contrast

From: Sean Hayes <Sean.Hayes@microsoft.com>
Date: Thu, 8 Mar 2007 23:44:00 +0000
To: WCAG-WG <w3c-wai-gl@w3.org>
Message-ID: <7261AC2A5F73904CA41773C8F00813FF1C67088D@EA-EXMSG-C309.europe.corp.microsoft.com>

Following up from our call, I thought I'd jot down a few of the issues around the proposed contrast provision.

HTML and CSS issues.

   Definition of background:
        In CSS the definition of background of an element refers to the background of the content, padding and border areas, each of which can have a different color, be transparent or be an image with some opacity. In addition it is possible for text and images to abut, or overlap with border areas and to overflow outside of these areas.

        So does the "background of text" mean background in the more limited css sense, or in the more general, "what the strokes of the glyph are drawn over in a specific rendering", sense?


        background-repeat may cause different strokes of the text to abut against different areas of the background depending on the size of the container and the size of the font.
        background-attachment = scroll/fixed can cause the background image to change with respect to the text in front of it as the user scrolls the window.

        The use of opacity in an image applied to content, padding and border, means computing a colour from the product of these three rectangles, as well as the elements behind the element. This number is not exposed by browsers; and may change with layout changes - including simple scrolling of the text.

        The definition is not testable with the use of system colors, since we don't know what the colors actually are.

        The clause for relaxing to 3:1 is based on font but the selection of font may be by the browser, so the height and stem width may not be known.

General content issues:

        Although HTML and CSS use sRGB, other content may not - for example many JPG images from digital cameras use AdobeRGB which has a different gamut, and so we need to define the mapping from other colour spaces into sRGB. Some material may be in colour separated CMYK, or Duotone forms.

        In general content it is possible to use richer text effects, such as text which is stroked and/or filled with gradients and images. The same issues of relative movement, e.g. due to scrolling would then apply.

        The 'averaging' technique for use with dithered backgrounds does not define the width or shape of the averaging window; or how to deal with text strokes that fall within the averaging window.

        Text and vector graphics in Flash moves of its own volition - and thus calculating contrast is time dependant.

Sean Hayes
Standards and Policy Team
Accessible Technology Group
  mob +44 7977 455002
  office +44 117 9719730
Received on Thursday, 8 March 2007 23:44:14 UTC

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