Manual:$wgDTD

Output: $wgDTD
Specifies the DTD in the DOCTYPE declaration for page output.
Introduced in version:pre 1.1.0
Removed in version:1.22.0 (Gerrit change 63106; git #97caae59)
Allowed values:(string)
Default value:'http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd' (1.3-1.21)
'http://www.w3.org/TR/html4/loose.dtd' (pre1.1 - 1.2)

Details edit

Specifies the DTD in the DOCTYPE declaration for page output.