At 05:18 PM 5/1/2007, Mike wrote:
> <!DOCTYPE foo SYSTEM "whatever" [
> <!ATTLIST foo xml:space #IMPLIED>
> ]>
> <foo xml:space="preserve">
> ....
>

Ah yes, that lovely feature of DTDs - you can do your own thing, and still
be valid.

This isn't a problem, as long as you define "validity" accordingly. 
(I.e., in a way that might nullify its practical usefulness in 
pathological cases. But what technology is immune from pathology?)



By making DTDs formally optional for parsing, XML took a big step 
away from SGML -- bigger than many engineers seem to have understood 
at the time. If MS had done the right thing at the start and not 
decided it was safe to throw away whitespace arbitrarily, this never 
would have been a problem. Even apart from xml:space (which is in 
essence a patch over a problem created when DTD parsing becomes 
optional), DTD content models offer valuable information about which 
whitespace is safe to throw away, which is why whitespace munging in 
the parse phase was thinkable under SGML. (Not that the MS parser 
uses this information. Its original developers seem not to have 
thought about mixed content.) But in XML, the DTD is not always 
there, and the decision of which whitespace is "significant" or not 
becomes subject to unpredictable interactions between document 
instances (which may or may not invoke DTDs) and processing 
architectures (which may or may not parse them even when invoked).



This is why well-behaved XML processors do not take it upon 
themselves to throw away whitespace without explicit say-so, and why 
XSLT had to go beyond its purview and hope, impossibly, to dictate 
upstream processor behavior: "Please don't throw away whitespace; I 
can take care of it".



Cheers,
Wendell


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Wendell Piez                            mailto:wapiez@xxxxxxxxxxxxxxxx
Mulberry Technologies, Inc.                http://www.mulberrytech.com
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