I added CSS I found
on
mediawiki.org to MediaWiki:Common.css:
<h3 id="siteSub"><?php $this->msg('tagline')
?></h3>
That's not CSS, that's HTML/PHP.
To get the tagline to show up, add this to [[MediaWiki:Monobook.css]]:
#siteSub { display: block; }
(You might need to add "!important" after the "block" to increase
specificity.)
HTH.
-- F.