Did HN's font change recently?
Am I crazy or did HN just change the font?
Another theory: during the Facebook outage, HN was experiencing a lot of extra load. I wonder if it's possible that your browser didn't fully download the necessary CSS files to display the site the way that you're used to -- and if maybe now it's still using that (cached) partially-downloaded CSS file?
Nothing's changed on my end, have you tried using a different browser, clearing your cache etc?
I disable remote fonts on my browser, so it just uses whatever font I enforce.