Manual:$wgAllowRdfaAttributes

Details
Enable RDFa attributes for use in wikitext.

For this to work Html5>Special:MyLanguage/Manual:$wgHtml5| must be set to , which is the default as well as unchangeable since MW 1.22+. Starting with MW 1.27+ RDFa attributes will be allowed by default. However the RDFa version to use must explicitly be set with Html5Version>Special:MyLanguage/Manual:$wgHtml5Version|.