JSON-LD vs Microdata vs RDFa, which should Magento use?

Google supports JSON-LD, Microdata and RDFa and recommends JSON-LD. JSON-LD can be easier to maintain separately from layout HTML, but existing correct Microdata or RDFa is not automatically obsolete. Inspect the theme and installed modules before adding another emitter. A syntax change cannot correct a wrong price, invented review or inconsistent entity.

References: Google structured-data introduction; Google structured-data guidelines.