You can configure that in LocalSettings.php
I have added the lines bottom to the LocalSettings, but no changes are
showed yet. I need restart something?
$wgDocType = '-//W3C//DTD XHTML+RDFa
$wgDTD = 'http://www.w3.org/MarkUp/DTD/xhtml-rdfa-1.dtd';
No, that should work.
Which version are you running? Maybe it's an old one which didn't
support such vars?