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SVG12: Meaning of "default value"

From: Bjoern Hoehrmann <derhoermi@gmx.net>
Date: Sat, 18 Mar 2006 23:15:57 +0100
To: www-svg@w3.org
Message-ID: <ffqo12t0qq77i3rl1p2ecs8jbebf937gv8@hive.bjoern.hoehrmann.de>

Dear Scalable Vector Graphics Working Group,

  http://www.w3.org/TR/2005/WD-SVGMobile12-20051207/ refers repeatedly
to the concept of "default value" but never explains the implications of
them. For example, the default value of the height attribute on the svg
element is described as "100%". I understand that implementations are
not required to reflect this default value in the DOM and that they are
required to use this value for the purposes of rendering. However, it is
not clear to me what the implications are for example when determining
the underlying value for 'to' animation. Simple test cases like

  <svg xmlns='http://www.w3.org/2000/svg' viewBox="0 0 100 100">
    <animate attributeName="height" to="50%" dur="3s" />
    <rect height="100" width="100" color="blue" />
  </svg>

  <svg xmlns='http://www.w3.org/2000/svg' viewBox="0 0 100 100">
    <animate attributeName="height" to="50" dur="3s" />
    <rect height="100" width="100" color="blue" />
  </svg>

  <svg xmlns='http://www.w3.org/2000/svg' viewBox="0 0 100 100">
    <animate attributeName="height" by="-50%" dur="3s" />
    <rect height="100" width="100" color="blue" />
  </svg>

behave in all sorts of strange and unexpected ways in current user
agents, it's rarely the same as when specifying from="100%" which is
what I would expect from the "default value" language. This is similar
for other attributes, though there aren't many well-supported ones
where the default value is neither a string nor 0. For the examples
above one authoring tool went as far as claiming SVG Tiny 1.1 does
not allow <animate> or the way it's used, which seems incorrect per
the Recommendation.

Please change the draft such that the implications of "default values"
are well-defined. Should default values affect the underlying value
even if they don't affect the DOM values, I think the SMIL 2.1 Re-
commendations needs to be amended to clarify that languages may have
such special rules. It is not clear to me what the implications of
unsupported values like <svg height="" would have on this either.

regards,
-- 
Björn Höhrmann · mailto:bjoern@hoehrmann.de · http://bjoern.hoehrmann.de
Weinh. Str. 22 · Telefon: +49(0)621/4309674 · http://www.bjoernsworld.de
68309 Mannheim · PGP Pub. KeyID: 0xA4357E78 · http://www.websitedev.de/ 
Received on Saturday, 18 March 2006 22:15:53 GMT

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