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[svg2] lacuna value definition (was: Re: SVG 2 FPWD published)

From: Erik Dahlstrom <ed@opera.com>
Date: Thu, 30 Aug 2012 11:01:14 +0200
To: "Jeremie Patonnier" <jeremie.patonnier@gmail.com>, "Chris Lilley" <chris@w3.org>
Cc: "Cameron McCormack" <cam@mcc.id.au>, "SVG public list" <www-svg@w3.org>
Message-ID: <op.wjujccdggeuyw5@gnorps>
On Thu, 30 Aug 2012 09:03:01 +0200, Chris Lilley <chris@w3.org> wrote:

> On Wednesday, August 29, 2012, 3:11:12 PM, Jeremie wrote:
> JP> I know it's a very early WD but here are some small quick thought  
> about it:
> Thanks, appreciated.
> JP> Section 1.6
> JP> I'm looking forward for the definition of "lacuna value" but I'm
> JP> not sure it is quite différente than "default value" which seams a
> JP> bit more easier to understand.
> Yes we probably do need to explain better; the terms are quite different.
> In XML, "default" values for attributes define "how an XML processor is  
> to react if a declared attribute is absent in a document".
> http://www.w3.org/TR/xml/#sec-attr-defaults
> Note that attribute-value normalization may add such default values to  
> the infoset (I say may because it depends on whether external DTD  
> subsets are fetched or not). It turn, this affects what is in the DOM,  
> whether CSS attribute selectors match or not.
> In other words a default value is missing in the document and *may or  
> may not* show up in the DOM, etc. This is an undesirable cause of  
> variability in implementations. It can also result in DOM bloat.
> The term lacuna value was introduced in SVG Tiny 1.2 to deal with this.
> http://www.w3.org/TR/SVGTiny12/intro.html#TermLacunaValue
> A lacuna value is missing in the document, is *not* added to the infoset  
> or to the DOM and can *not* be matched by attribute selectors. The  
> specification tells the implementation what to do if tthe value is  
> missing, without needing to sort-of-maybe add the value to the document.

I don't know if we have a need for it (yet), but perhaps we should  
consider splitting the lacuna value definition in two such that we get one  
term invalid-specified-value-lacuna, and one term  
unspecified-attribute-lacuna for when an attribute is not specified?

A construct like that might be nice if one wants to make the UA do one  
thing if the attribute was specified (but value invalid), and another if  
the attribute wasn't specified at all. This relates to the  
radialGradient@fr discussion. The point would be to indicate that you're  
doing new svg2 stuff by the existence of the attribute itself, and the  
other half (unspecified-attribute-lacuna) is for backwards compat.

Just a thought...
Erik Dahlstrom, Core Technology Developer, Opera Software
Co-Chair, W3C SVG Working Group
Personal blog: http://my.opera.com/macdev_ed
Received on Thursday, 30 August 2012 09:02:18 UTC

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