Issue
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The expected behaviour when a processor encounters a document
containing markup which it does not understand is not clear.
Proposal
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A processor MAY validate documents against the standard schema and MAY
reject any which do not conform. For any document that a processor does
not reject as invalid, any markup that the processor cannot understand
by reference to this specification MAY be discarded.
I think this is a reasonable statement to add, however, one tweak is needed -
the example schema are informative, not "standard".
The real question is where to put this text. There doesn't seem to be
an existing section where it fits so I added a 4.3 "Validaion" section
to the document.
Rationale
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I think it is better to say something explicit about this issue. This
proposal is not one that I particularly favour but is the de facto status
quo as I see it. It is not unreasonable.
I agree with all of this.
Ned