The site specifies a base font size of 12px. The better practice is to not specify a base font size at all, just taking it from the user's web browser instead. Then, the web designer should specify every other font size and box dimension as a scaled version of the base font size, using units like em/rem/%, not px.

Related reading: https://joshcollinsworth.com/blog/never-use-px-for-font-size

It's the same size as HN: 12px. HN looks larger to me for some reason, but I can't figure out why: when I overlay a quote someone posted here over the website with half transparency in GIMP, the text is clearly the same height. Some letters are wider, some narrower, but the final length of the 8 words I sampled is 360px on HN vs. 358px on that website (so differences basically cancel out)

This is on Firefox/Debian, in case that means something for installed fonts. I see that site's CSS specifies Verdana and Arial, names that sound windowsey to me but I have no idea if my system has (analogous versions to) those

There's a PDF version linked at the top of the article, it's actually much better typeset.

Given the subject of TFA, this seems appropriate in a meta sort of way.