[David Carlisle] Re: question about xlink 1.1

attached mail follows:

Henry,

I don't think I can post to public-xml-core-wg but a comment on

http://lists.w3.org/Archives/Public/public-xml-core-wg/2009Jun/0030.html


> However, I am left with one residual query in this area:  why isn't
> href #REQUIRED in simpleAttrs in the DTD (and schemas)?  Why does the
> spec. say "it MAY be used on simple-type elements"?  What does a
> simple link w/o an href mean?

a dtd fragment using #IMPLIED  is more useful than one using #REQUIRED
for many uses of xlink (and for similar reasons the dtd shouldn't
default a type="simple" attribute, now that xlink 1.1 doesn't require
one.


Consider for example MathML2 where every element can be used as a link,
by using xlink:href attribute. It's useful to be able to include a dtd
fragment that sets up #IMPLIED xlink attributes so that this usage is
DTD valid but you can't _require_ href on every element.

Defaulting type="simple" is slightly different. xlink 1.0 mandated use
of type attribute so mathml originally defaulted type="simple" in the
dtd which looked fine from an authoring point of view, but we got very
negative feedback from mozilla (in particular) because if they used a
dtd that defaulted this attribute it was really there in the DOM on
every element and in a markup heavy language like mathml that is a heavy
cost, especially if the majority of documents are not using xlink at
all.  So at some point we changed the dtd not to default type="simple"
which meant it had to be explict on every link. So xlink 1.1 not
requiring this would be a good thing, but not using a dtd that defaults
the attribute again.


Feel free to forward or discard as you see fit:-)


David

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       Henry S. Thompson, School of Informatics, University of Edinburgh
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