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Re: [css-variables] using CSS Variables as a mechanism for parameterised content

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Thu, 19 Mar 2015 13:00:30 -0700
Message-ID: <CAAWBYDCaB6VQBA2zB35hDaZ+cheKXYLEWbSF5T9A4g1kkiYtjQ@mail.gmail.com>
To: Cameron McCormack <cam@mcc.id.au>
Cc: "www-style@w3.org" <www-style@w3.org>
On Wed, Sep 3, 2014 at 10:01 PM, Cameron McCormack <cam@mcc.id.au> wrote:
> In the OpenType specification, CSS Variables are used to pass in colour
> palette values into SVG documents that can be used to define glyphs. This is
> done by having the implementation apply a user agent style sheet that looks
> like:
>
>   :root {
>     --color0: rgba(...);
>     --color1: rgba(...);
>     ...
>   }
>
> so that you can then reference those variables in fill and stroke
> properties.  When I proposed this, it seemed like a natural fit.  It also
> looks like what you'd want when we eventually get around to supporting
> parameterised SVG documents (however we end up allowing explicit values to
> be passed in).
>
> I've come to realise now though that this has a distinct disadvantage:
> because the computed values on the root element, and any elements that
> reference the variables, are different depending on the particular colour
> values being passed in, an implementation cannot easily have a single
> document loaded and referenced with different parameter values. Well, it
> could, but it would mean that each time you want to render the document
> you'd need to restyle it with the updated user agent style sheet.
>
> This makes me wonder whether we should have a way to reference values from
> the outer document without having the particular values be reflected in the
> computed value of the property that's referencing them.  I don't have any
> concrete suggestions yet; Doug's SVG Parameters proposal used
> "param(some-name)" as the syntax.  The important part is that you could
> write say <path style="fill: param(some-name)"> and the computed value of
> 'fill' would be "param(some-name)", not the value of the some-name
> parameter.  This would allow the implementation to look up the value at
> rendering time, and avoid having to restyle the document if you have
> multiple references to the document using different colours.
>
> Note that this works well for <color>/<image> values, as changing them would
> not require the document to be reflowed.  If we want to support
> parameterisation of other property types using a mechanism other than CSS
> Variables, I'm not sure whether we could make any improvement.
>
> I'm interested to hear others' thoughts.

Responding here to finish up my Variables DoC. ^_^

We discussed this at the last FXTF f2f
<https://lists.w3.org/Archives/Public/public-fx/2015JanMar/0065.html>.
There wasn't a good conclusion, except that maybe browsers can just
detect that a variable used for a font palette is given a color value
and just used for color things, so it can just apply the variable
color as a rendering-time effect.  I think that's about as good as we
can do within the Custom Properties framework; addressing it properly
requires a fairly different feature with very different tradeoffs.

~TJ
Received on Thursday, 19 March 2015 20:09:07 UTC

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