Flash of unstyled content on Firefox

When browsing the blog on Firefox, there's a visible flash of unstyled content (FUOC), and a warning message in the browser console:

Layout was forced before the page was fully loaded. If stylesheets are not yet loaded this may cause a flash of unstyled content.

I'm not sure why this happens, because there isn't any javascript at all on the front page, and yet it still happens there.

We should check if the CSS is causing this maybe?