Re: [CSS21] as-rendered properties

On Monday 2005-08-29 15:31 -0400, Al Gilman wrote:
> At 2:58 PM -0700 7/21/05, L. David Baron wrote:
> >On Thursday 2005-07-21 17:38 -0400, Al Gilman wrote:
> >> at the CSS working group. What provision is being made in the CSS DOM
> >> API to acquire the final format css properties. This information is
> >> used by assistive technologies and in fact is a fulfillment
> >> requirement of accessibility APIs. It is valuable for platform
> >
> >How is the getComputedStyle method in
> >http://www.w3.org/TR/2000/REC-DOM-Level-2-Style-20001113/css.html#CSS-ViewCSS
> >insufficient for this requirement?
> 
> The assistive technologies need to know the actual values of 
> presentation properties.
> 
> It is our understanding that the computed style may not reflect the 
> property values
> that actually appear in the user experience.  Is this true?

That is true.  For example, when a device has bitmap fonts available at
9, 12, and 15 pixels and the computed value of 'font-size' is 11px, the
12px font might be used (or, likewise, if the computed value is 11.8px
and the device or graphics API is only capable of rendering fonts at
integral pixel sizes).  Or if the computed value of 'color' is #A0BDEA
and the device has 8-bit color, the color #99CCFF might be used instead.

> http://www.w3.org/TR/CSS21/cascade.html#actual-value
> 
> When they differ, we need the actual values, as opposed to the computed 
> values.

Given the examples I gave, I'm not quite sure why.  Do you have some
examples showing why?

Requiring implementations to *know* the actual value in some of these
cases might be a significant burden (and not one that we currently
impose).  Consider the examples of printing, where an implementation
might not know whether it's printing to a color or black and white
printer, or an implementation built on top of a graphics or font API
where it might not know what approximations the API it's using needs to
make.

> Thank you for you assistance in clarifying this comment.

It seems to be a reasonable comment on the CSSOM specification (although
I still think I disagree, based on what I've heard so far), but I still
don't see how it's a comment on CSS2.1.

-David

-- 
L. David Baron                                <URL: http://dbaron.org/ >
          Technical Lead, Layout & CSS, The Mozilla Foundation