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?

Did you recently update your OS? I had to disable font smoothing on my Mac to avoid blurry text.

https://colinstodd.com/posts/tech/fix-macos-catalina-fonts-a...

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.