By using serif fonts for headings and sans-serif fonts for bodies they have broken the page alignment: the serifs push the headings over by enough pixels to be really noticeable; this kind of design requires really careful attention to optical margin layout (which is, of course, really problematic in CSS).
If this is interesting, you might also like to read the longer post by Wikimedia's Director of UX about the change. <a href="https://medium.com/p/1724cb2b7402" rel="nofollow">https://medium.com/p/1724cb2b7402</a>
I thought I was crazy when I popped in to check something earlier. Bolds look a bit lopsided, especially at small sizes. But it's probably a positive change overall.