Schematron
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Schema language based on patterns instead of grammar
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Suitable for more "relaxed" markups, where it is enough to specify the schema only "roughly" by setting some rules, not exact grammar exhaustively
Examplotron
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Examplotron was an experiment to define markup model by just giving example(s) of valid document(s)
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The "example document" is converted to a RelaxNG schema and then the normal RelaxNG validation can be done.
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Not widely used but simple to use with no major ostacles.