Validating schema online
RELAX NG is also an International Standard (ISO/IEC 19757-2).
It is Part 2 of ISO/IEC 19757 DSDL (Document Schema Definition Languages), which is maintained by ISO/IEC JTC1/SC34/WG1.
Defines annotations and datatypes that support some of the features from XML 1.0 DTDs not supported directly by RELAX NG. XML Matters: Kicking back with RELAX NG, Part 1, Part 2, Part 3, David Mertz, IBM developer Works.
However, I want to know about an official and formal statement regarding why in an XML document it is OK to omit the URL for the XMLSchema-instance namespace and probably an implementation advice that parsers can or should use a cached local version of its XSD for validating XML docs. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
Would you like to answer one of these unanswered questions instead?