You can find the XML Schema for Schemas here.
However, you probably do not need it unless you’re trying to validate an XSD (as opposed to validating an XML document instance with against an XSD).
- xmlns, xmlns:xsi, xsi:schemaLocation, and targetNamespace?
- How to link XML to XSD using schemaLocation or noNamespaceSchemaLocation?
- Must an XML namespace name URI be retrievable?
- What is the difference between xsd and xsi?
- How to restrict the value of an XML element using xsi:type in XSD?