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[Fwd: CSS 2.1, section 5.8.2]

From: Bert Bos <bert@w3.org>
Date: Wed, 24 Dec 2003 21:17:09 +0100
Message-ID: <3FE9F445.50701@w3.org>
To: www-style@w3.org



-------- Original Message --------
Subject: [Moderator Action] CSS 2.1, section 5.8.2
Date: Tue, 23 Dec 2003 17:29:34 -0500 (EST)
From: C. M. Sperberg-McQueen <cmsmcq@acm.org>
To: www-style@w3.org
CC: W3C XML Coordination Group <w3c-xml-cg@w3.org>



Dear colleagues:

The XML Coordination Group congratulates you on the progress of the
CSS 2.1 specification.

We write you with some concern, however, about one detail of the
specification, which may raise an important inter-Activity
coordination issue.

It has been called to our attention that in section 5.8.2 "Default
attribute values in DTDs" [1], the CSS 2.1 specification is subject to
two different and contradictory readings.

[1] http://www.w3.org/Style/Group/css2-src/diffs-rec/selector.html#q11

The words

     Default attribute values may be defined in a DTD or elsewhere, but
     cannot always be selected by attribute selectors. Style sheets
     should be designed so that they work even if the default values
     are not included in the document tree.

may be taken as meaning either

(1) Because not all XML parsers read an external DTD, it will
     occasionally be the case that a CSS process does not, in practice,
     have access to defaulted attribute values; it is thus wise to take
     special steps to ensure that the stylesheet works as desired even
     if default attribute values are not accessible to the process.

or

(2) Because not all XML parsers read an external DTD, there will
     be cases when CSS processes do not have access to defaulted
     attribute values.  For this reason, and in order to ensure that
     all CSS processors produce identical results even if some use
     validating XML parsers and others use processors which ignore the
     external DTD even for non-standalone documents, a CSS process is
     forbidden to process attributes if the attribute value was
     supplied by default from a DTD, schema, or other external
     information.

We hope that interpretation (1) is the intended one, and suggest that
the wording of section 5.8.2 be modified to make that interpretation
more explicit, and to rule out interpretation (2).  One possible
change might be to modify the first sentence quoted above, while
retaining the second unchanged:

     Default attribute values may be defined in a DTD or elsewhere, but
     will be available for selection by attribute selectors only if
     the XML parser in use processed the DTD or other source of
     information.  Style sheets should be designed so that they work
     even if the default values are not included in the document tree.

We believe it would be a mistake to allow interpretation (2) to take
hold in the implementor or user communities.  There are XML
vocabularies which make extensive use of defaulted attributes, and
there are implementations that can do useful things with them; it
would be regrettable were the use of default attributes to be made, in
effect, incompatible with CSS.  Equally bad would be the consequence
of making DTD- and schema-aware processors unusable as the upstream
XML parsers for CSS processing.  Most such processors do not
distinguish in their output between attributes whose value was
supplied explicitly in the input data stream and attributes whose
value was supplied by default; interpretation (2) would make it
non-conforming for any CSS processor to use such an XML parser.  We
think that would be a mistake both because it would needlessly
restrict the choice of XML parser to be made by implementors of CSS,
and because it would work against use of DTDs or schemas in material
intended to be rendered using CSS.

We hope that you will, on reflection, agree that interpretation (1) is
preferable to interpretation (2), and revise the text accordingly.
Please let us know whatever you do.

Respectfully,

C. M. Sperberg-McQueen
on behalf of the XML Coordination Group




-- 
   Bert Bos                                ( W 3 C ) http://www.w3.org/
   http://www.w3.org/people/bos                               W3C/ERCIM
   bert@w3.org                             2004 Rt des Lucioles / BP 93
   +33 4 93 65 76 92               06902 Sophia Antipolis Cedex, France
Received on Wednesday, 24 December 2003 15:17:29 GMT

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