Re: [css3-background] PFWG Comments: high contrast modes

On 10/27/2010 09:36 AM, Michael Cooper wrote:
> The following is input from the Protocols and Formats Working Group on
> CSS 3 Backgrounds and Borders. Approval to send this as a WG comment is
> archived at http://www.w3.org/2010/10/27-pf-minutes.html#item08.
>
> PF has concerns about the use of CSS background-image by authors to add
> images which are not backgrounds to pages. Authors do this to take
> advantage of positioning and clipping features for CSS which are not
> available with the HTML img element. One reason for this is to allow
> multiple graphics to be combined in a single file to reduce server
> round-trips, which can substantially reduce the monetary cost of
> maintaining sites by reducing the number of servers needed and the
> amount of bandwidth used. Because of the monetary costs involved, we do
> not believe it is realistic to ask authors to discontinue this practice.
>
> Accessibility accommodations for users with low vision often disable
> background colors and images in order to display text more visibly.
> These accommodations include High Contrast settings in the Windows and
> Macintosh operating systems, accessibility settings in several browsers,
> and assistive technologies. Disabling backgrounds is the correct
> behavior for these tools, but disabling meaningful images is not.
>
> With the current CSS implementation, there is no way for authors to
> indicate which CSS images should be removed in low vision scenarios and
> which should not. The PF working group feels strongly that there needs
> to be a mechanism for authors to indicate this.

The mechanism that exists in CSS is to use the 'content' property
for foreground images and the 'background' property for background
images.

With regards to CSS spriting, the CSSWG plans to adopt the Media
Fragments WG's syntax for extracting a portion of the image:
   http://www.w3.org/TR/css3-images/#url
   http://www.w3.org/TR/media-frags/#naming-space
This can be used with the 'content' property (and also with the <img>
element in HTML for content images).

> We have three possible solutions to propose:
> 1) foreground-image: Foreground images sit on top of background images,
> ...
> Any of these solutions would be acceptable. In any case, we would also
> suggest referencing WCAG 2.0 SC1.1.1, specifically F3 (Failure of
> success criterion 1.1.1 due to using CSS to include images that convey
> important information).

I have added the following paragraph in the 'background-image' definition:

<p>For accessibility reasons, authors should not use background images
as the sole method of conveying important information.
See <a href="http://www.w3.org/TR/2008/NOTE-WCAG20-TECHS-20081211/F3">Web
Content Accessibility Guideline F3</a> [[WCAG20]]. Images are not
accessible in non-graphical presentations, and background images
specifically might be turned off in high-contrast display modes.

Please let me know if this addresses your concerns.

~fantasai

Received on Wednesday, 27 October 2010 17:40:42 UTC