Manual:$wgHtml5Version

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

If you use RDFa, the possible values are:



Before MediaWiki 1.27, if you enable , this setting will be auto-initialized to the correct value for RDFa+HTML5.

Starting with MW 1.27.0 you don't need to specify this setting if you want to use RDFa, since it's enabled by default.