Manual:$wgAllowRdfaAttributes

Details
Enable RDFa attributes for use in wikitext.

For this to work  must be set to, which is the default as well as unchangeable since MW 1.22+.

Starting with 1>phabricator:rMWe789f51f1183d57999926b80c726338d8611e2f1|MW 1.27+ RDFa attributes will be allowed by default.

However the RDFa version to use must explicitly be set with .