W3C home > Mailing lists > Public > www-html@w3.org > April 2001

RE: Seprarately Bracketing Attributes

From: Dave J Woolley <david.woolley@bts.co.uk>
Date: Wed, 18 Apr 2001 11:51:57 +0100
Message-ID: <81E4A2BC03CED111845100104B62AFB50102A63D@STAGECOACH>
To: W3C HTML <www-html@w3.org>
> From:	Russell O'Connor [SMTP:roconnor@math.berkeley.edu]
> > it would probably make it impossible to parse a document
> > without the DTD, which is one of the primary diffferences
> > between XML and SGML.
> XML is SGML.  SGML doesn't require a DTD.
	You cannot parse general SGML without knowledge
	of the DTD, whether or not that DTD is a formal
	DTD or just a narrative description.  That's 
	fairly easy to demonstrate on an HTML page with
	a broken document type as the W3C validator will
	attempt to do just that and will fail whenever it finds
	an element with an optional and omitted closing tag -
	it assumes that the closing tag was not optional and
	fails to close any elements subsequently

	Another example of where SGML cannot be accurately parsed
	without a DTD is style and script elements in HTML, which 
	have a CDATA content model.  Without knowing that they are
	CDATA, the parser cannot know that it should not parse them.

	XML, on the other hand requires that all elements be
	explicitly closed and that CDATA sections be explicitly 
	marked as such on every occurrence.  Both of these are 
	intended to ensure that you can create a correct parse tree
	without knowledge of the DTD (it also doesn't allow optional
	start tags, which is another thing that needs the DTD for a 
	correct parse of general SGML).

	SGML doesn't require a <!DOCTYPE, but the parser still needs to
	be aware of the underlying DTD.

	Whilst XML is probably technically compliant with SGML, it relies
	on a processing instruction to allow it to have features which
	look a bit like SGML's default concrete syntax, but are actually
	radically different - e.g. XML DTDs are not valid SGML DTDs.

	I'm pretty sure that the sp tools can only handle XML as a result
	of hard coded special handling for XML, rather than simply using
	the DTD and specification.
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Received on Wednesday, 18 April 2001 06:51:54 UTC

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