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[css3-background] PFWG review of css3-background 2009-10-15 LCWD

From: Michael Cooper <cooper@w3.org>
Date: Wed, 06 Jan 2010 12:53:45 -0500
Message-ID: <4B44CE29.7000608@w3.org>
To: www-style@w3.org
CC: List WAI Liaison <wai-liaison@w3.org>
The following is review comments from the Protocols and Formats Working
Group on CSS 3 Backgrounds and Borders, Last Call Working Draft of 15
October 2009 <http://www.w3.org/TR/2009/WD-css3-background-20091015/>.
We are aware these comments are late (due to process difficulties on our
end) and apologize for that, and will take our delay into account in how
we address your responses. PFWG approval to send these comments was
obtained 6 January 2010 http://www.w3.org/2010/01/06-pf-minutes.html#item05.

The basic issue is that in low-vision scenarios, CSS backgrounds are
often disabled. This is correct behavior, because in these scenarios,
the user agent changes all the colors, and removes background
decoration, gradients, etc. in order to improve readability. This
happens with Operating System High Contrast settings, and is also done
by some assistive technology.

Many Web developers are using CSS to apply images that are necessary for
understanding and operating a Web site, for example, a graphic that
defines a button or other clickable area.  The developers are doing this
to improve performance, by adding all the graphics for a page to a
single image file, and using CSS positioning and clipping to apply to it
various UI elements, thereby reducing the number of http requests.  This
approach is sometimes called ‘sprites’.  Developers who use sprites are
often unwilling to give up the performance in order to provide
accessibility to low visions users, as doing so can often have rather
significant monetary costs (additional servers, increased bandwidth
consumption, etc).

Users who use high contrast settings or AT with similar behavior, will
not see sprites, and the site will not be accessible to them. 

What is needed is a mechanism that allows developers to indicate that a
particular application of an image by CSS is a sprite, and should not be
removed when backgrounds are removed.  It would also be nice to have a
mechanism to provide different graphics to these users, such as graphics
drawn in high contrast.

Here’s one suggestion of how to address this issue.

foreground-image:  Foreground images sit on top of background images,
but are otherwise identical.  They are semantically different, in that
they are images that are intended to be the only rendered representation
of the element.  Foreground images MUST NOT be disabled by assistive
technology.

foreground-image-high-contrast:  If this property is specified, this
image MUST replace the foreground image when the user agent is rendering
in high contrast mode, in response to operating system, browser, or
assistive technology settings.

background-image-high-contrast:  If this property is specified, this
image MUST replace the background image when the user agent is rendering
in high contrast mode, in response to operating system, browser, or
assistive technology settings. 

-- 

Michael Cooper
Web Accessibility Specialist
World Wide Web Consortium, Web Accessibility Initiative
E-mail cooper@w3.org <mailto:cooper@w3.org>
Information Page <http://www.w3.org/People/cooper/>
Received on Wednesday, 6 January 2010 17:53:49 GMT

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