Glossary URI — defined by RFC 3986 IRI — defined by RFC 3987 Formats Google intro to structured data JSON-LD Microdata RDFa I’ve opted to use both JSON-LD and Microdata, and ignore RDFa which seems to have fallen out of fashion with XML.
The reason I’m using Microdata is I’ve found it an aid in keeping my HTML templates logical and neat, though it does sometimes confuse Google’s scrapers in that two different Metadata formats on the same page invites contradictions.