Manual:$wgHtml5Version

Details
Defines the value of the version attribute in the   tag, if any.

If you use RDFa, the possible values are:



If you forget to specify this setting, MediaWiki automatically sets it if you enable $wgAllowRdfaAttributes (≤ MW 1.26.x). Starting with MW 1.27.0 one must specify this setting if you want to use RDFa.