forgot to mention that it is mobile Firefox.

I had checked the settings earlier, nothing seemed applicable.

interestingly, just now I tried it on mobile Chrome, on the same phone, and it rendered okay.

https://pasteboard.co/pPrVKF3dyyhz.png

(didn't think of that earlier somehow, dumb me, maybe because the page wasn't that important to me.)

so it may be a mobile Firefox issue.