Re: [xml-dev] Victory has been declared in the schema wars ...

[Henri Sivonen <hsivonen@iki.fi>]

On Nov 28, 2006, at 14:21, David Carlisle wrote:

> Of course this rng based xhtml2 hasn't proved to be overwhelmingly,
> popular, before declaring victory we'd better keep an eye on html5  
> which
> manages to do without dtd or xsd or rng, or any other recognised  
> schema
> language at all.....

I think not having a normative schema for HTML5 is a very good idea.  
None of the current schema languages (not even Schematron) are  
adequate for expressing all the conformance requirements of HTML5.  
Experience has shown that people become too schema-focused if there  
is a normative schema: DTDs aren't adequate for expressing the  
conformance requirements of HTML 4.01, but still people behave as  
though they had checked for conformance when they have validated  
against a DTD.

Still, HTML5 can be counted as a bullet point in favor of the RELAX  
NG camp. The only schema for HTML5 in the works is a RELAX NG  
(Compact Syntax) schema with companion Schematron assertions ( http:// 
syntax.whattf.org/ ). A conformance checker for HTML5 requires non- 
schema checking code to fill the gaps that RELAX NG and Schematron  
can't fill (conveniently or at all). For example, http:// 
hsivonen.iki.fi/validator/html5/ checks for table cell overlap in  
Java code--not in RNG, XSD or Schematron.

But back to RNG vs. XSD:
Dare Obasanjo pointed out an important issue on his blog at
http://www.25hoursaday.com/weblog/PermaLink.aspx? 
guid=19adeb36-16d9-4cb6-a99f-cf3d1b15de5c
Validation and data binding are different problems.

RELAX NG is great for validation. For this purpose, allowing  
ambiguous grammars and making datatypes just check whether a string  
belongs in a formal language are great features. Trying to introduce  
something as innocent-looking as the RNG DTD compat features already  
becomes an annoyance that gets in the way of writing nice ambiguous  
schemas. Infoset augmentation as a side effect of validation is not  
cool.

A lot of people use XSD for data binding. RNG doesn't do data binding  
*by design*, so telling those people to use RNG for data binding  
doesn't seem to help. Personally, though, I have serious doubts about  
whether automated data binding is a good idea--in particular due to  
service versioning concerns. And I am not suggesting that XSD is a  
good solution for data binding, either.

P.S. Readers of this thread may be interested in an online RNG and  
Schematron 1.5 validator:
http://hsivonen.iki.fi/validator/

-- 
Henri Sivonen
hsivonen@iki.fi
http://hsivonen.iki.fi/