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Re: SVG12: Vendor-specific presentation attributes

From: Chris Lilley <chris@w3.org>
Date: Thu, 6 Apr 2006 13:53:22 +0200
Message-ID: <1889138833.20060406135322@w3.org>
To: www-svg@w3.org
Cc: Bjoern Hoehrmann <derhoermi@gmx.net>

Hello www-svg,

 
> * Chris Lilley wrote:
>>BH>   http://www.w3.org/TR/2005/WD-SVGMobile12-20050413/struct.html does
>>BH> not seem to include adequate requirements for vendor-specific
>>BH> presentation attributes even though these apparently may be defined
>>
>>We are unable to discover anywhere in the specification that allows a
>>custom, vendor-specific property to be defined.
> 
> Are presentation attributes and properties the same thing? If not your
> response seems a bit odd.

They are related, but not the same thing. Properties are defined in the
specification. New properties cannot be defined by the document author.
(The requirements were we to add such a facility in the future were
listed later in my original reply).

Presentation attributes are one way to set the value of a property.
There are other ways, for example using CSS stylesheets, or using the
DOM.

Which attributes are presentation attributes is defined in the
specification. New presentation attributes cannot be defined by  the
document author.

Attributes are, well, attributes. As long as they are not in the default
namespace on svg elements, new attributes may be defined by the document
author, and in fact this is commonly done. However, that does not make
them presentation attributes and does not produce any link between that
attribute and any new or existing property.

See the section 'Foreign namespaces and private data' in the
Extensibility chapter for details.


>>BH> and implemented in conforming implementations. Please change the
>>BH> draft such that either this is not allowed (e.g., only "attributes"
>>BH> not "properties" may be used for extensions)
>>
>>This is the current situation. Furthermore, such attributes must be
>>qualified, if they are on an element which is in the SVG namespace. So,
>>we believe the specification to already be clear in this regard.
>>
>>BH>  or such that it is
>>BH> required that such properties must conform to
>>BH> http://www.w3.org/TR/CSS21/syndata.html#q4
>>
>>Although interesting, and perhaps something to consider for a future
>>specification, such a method would require a means to declare:
>>
>>- that the attribute is in fact a property
>>- whether it is inheritable or not
>>- the value space
>>- use of percentages
>>- computed value
> 
> So you are saying

What we are saying is listed above. What you are saying is below, and
does not correspond to what we are saying since you did not understand
the difference between properties, presentation attributes, and
attributes. Please review these definitions; they are connected, but not
identical by any means.

> that it is neither possible nor allowed to come up
> with presentation attributes

correct

> like a:adobe-knockout

you do not demonstrate that this is a presentation attribute. Its an
attribute; its not in the svg namespace or the default namespace.

> and corresponding properties like adobe-knockout,

you gloss over how this is a 'corresponding' property.

>  and that this is obvious from the current draft.

Yes.

> I don't see evidence for any of that, so this does not
> address my concern at all.

We do not find your argument convincing. You refer in vague terms to
creating presentation attributes and creating 'corresponding' properties,
but do not explain how that would be possible. We have explained both
that it is not possible, and also what would be required to make it
possible in a later version of the specification.

In CSS stylesheets it is certainly possible to reference vendor-specific
properties, such as -vendor-specific but SVG Tiny 1.2 does not allow
author stylesheets. CSS is also silent on how the initial value, the
inheritability, the computed value,and the handling of percentages is
communicated to the application.

In SVG Full, author CSS stylesheets are allowed, and thus
vendor-specific properties would be allowed also, but there is still no
way, in the current specification, to create a mechanism that would
allow these properties to be associated with a corresponding attribute
such that the attribute became a vendor-specific presentation attribute.



-- 
 Chris Lilley                    mailto:chris@w3.org
 Chair, W3C SVG Working Group
 W3C Graphics Activity Lead
 Co-Chair, W3C Hypertext CG
Received on Thursday, 6 April 2006 11:53:26 GMT

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