I see no thread about this and am not sure what else to do about it, so here goes:
All the text on http://boingboing.net/blog is boldened at the moment when viewed in Firefox. View the page source, and it is reasonably helpful about pointing out where the error lies:
Strangely, this doesn’t show up in the Inspector, where the tag is properly closed. Internet Explorer 11 isn’t bothered by it at all.
This started yesterday, sometime around when the Sanders article appeared.