I made a MediaWiki extension here that adds tags that can be used to format stories with larger spacing and fixed width so they don't turn into giant walls of text.
http://www.mediawiki.org/wiki/Extension:Story_Paragraph
I'm recommending this since it could be useful here. One of the reasons I created it is because it's shorter and more user friendly than creating a CSS class that does the same thing, since setting the class requires some HTML knowledge and isn't intuitive.
Automatically Appended Next Post: Actually, it seems it's escaping wiki markup.
Automatically Appended Next Post: On my own computer, I removed htmlspecialchars() from $output in a similar extensions. I tried adding an iframe in the tags to a personal wiki and it seemed to be escaped, presumably with the rest of the wiki text. I haven't altered the published code yet since I want to make completely sure that what I did was the right solution.