RDF/XML

Example

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:schema="https://schema.org/">
  <schema:WebSite>
    <schema:name>Schema Markup</schema:name>
    <schema:url rdf:resource="https://schemamarkup.app/" />
  </schema:WebSite>
</rdf:RDF>

References

Specifications: Extensible Markup Language (XML) 1.0 (Fifth Edition); RDF 1.1 XML Syntax
Media Type: application/rdf+xml (or application/xml)
File Name: *.rdf (or *.xml)