Why are links at the top, view counts, tags, comments, etc. italicized?

Starting yesterday, almost everything on Maths.SE (and other SE sites, too, I might add) is italicized:

This behaviour occurs in Chrome (30.0) as well as Opera (12.16). It does not seem to occur in Firefox (25.0.1).

Refreshing, clearing cache, incognito browsing, nothing works.

(One can check if the Arial Regular font is present by navigating to Control Panel > Fonts > Arial (at least on Windows 7).)