Manual:$wgAllowRdfaAttributes/en

Details
Enable RDFa attributes for use in wikitext. For this to work $Html5 must be set to, which is the default as well as unchangeable since MW 1.22+. Starting with MW 1.27+ RDFa attributes will be allowed by default. However the RDFa version to use must explicitly be set with $Html5Version to  or.