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[cssom] Property Value API Ideas

From: Anne van Kesteren <annevk@opera.com>
Date: Mon, 30 Nov 2009 09:52:26 +0100
To: "CSS WG" <www-style@w3.org>
Message-ID: <op.u369lojd64w2qv@anne-van-kesterens-macbook.local>
During TPAC some members of the CSS WG and people outside the CSS WG  
discussed the idea of a potential replacement for CSSValue. Inspiration  
came from a very old proposal by Ian Hickson which was originally  
Member-only but I've since made available publicly here:


The idea is to make the members of CSSStyleDeclaration return a mix  
between DOMString and a real Object. This would mean that e.g.  
triple-equality checks would no longer work and hopefully nobody relies on  
those specifics but if they do we have to think of something else.  
(Suggestions for "else" that floated around were having a new member on  
CSSStyleDeclaration that gives a CSSImprovedStyleDeclaration, having a new  
member for each CSS property, etc.)

I think that if we go in this direction we should start out small. Every  
member (width, cssFloat, etc.) of CSSStyleDeclaration would start  
returning the new DOMString/Object construct but only a few of them would  
return specialized constructs. Obvious candidates are properties that  
return some variant of <length> and properties that return some variant of  
<color>. Personally I'd also like one for <uri> that gives you the value  
without "url(" and ")" around it.

The problems with exposing a better API like this is that the syntax of  
CSS is very dynamic. So to preserve backwards compatibility at the  
API-level we might have to sacrifice some of the flexibility we have at  
the syntax-level or we have to get very creative (and potentially ugly) at  
the API-level. For instance, background-image was a single value at one  
point, but now it is a comma-separated list. E.g. if we define that  
list-style-image returns a CSSUrlConstruct and later decide that it gets a  
comma-separated list as well there is a small issue there. In that  
obj.style.listStyleImage.url still has to function properly.

The other problem I see is with properties that become a shorthand later  
on. E.g. if we turn background-position in a shorthand we might get some  
API breakage as well. Though this is probably already an issue given that  
in most implementations shorthand properties will always return the empty  

Any ideas and thoughts on this are very much appreciated!

(To avoid potential confusion, the section "CSS Property Value APIs" in  
the current CSSOM editor's draft is to be renamed to something like  
"DOM-Level Access to CSS Declaration Blocks" to more accurately reflect  
what it is about.)

Anne van Kesteren
Received on Monday, 30 November 2009 08:53:07 UTC

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