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   Re: [xml-dev] XPath 1.5? (was RE: [xml-dev] typing and markup)

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Jeni Tennison wrote:

> There are several aspects of HTML that can't be validated by
> grammar-based schema languages, such as the fact that form elements
> shouldn't occur within other form elements at any level [...]

The HTML DTD specifies this constraint with an exclusion exception
(a feature that was left out of XML):

    <!ELEMENT FORM - - %body.content -(FORM) +(INPUT|SELECT|TEXTAREA)>

TREX can also express it concisely with a combination of CONCUR
and NOTALLOWED.  CONCUR was left out of RELAX-NG, but it can also
express this constraint, as can RELAX and any other schema language
based on tree-regular grammars.

--Joe English



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