Centering Items

What would be the best way to center something like a diagram?

Is there any syntax available like for HTML?

There is not exactly wiki syntax just for that, but you can produce a div with pretty much any css through XWiki group syntax: see last example on https://www.xwiki.org/xwiki/bin/view/Documentation/UserGuide/Features/XWikiSyntax/?syntax=2.1&section=Groups.

Of course, at worst, you can always simply use the html macro to insert any html you want.