[generuso]

The author is Peter Bloem, and the html is compiled from these sources: https://github.com/pbloem/gestalt.ink

with the help of mathjax: https://www.mathjax.org/

The font seems to be Georgia.

[creata]

The CSS says:

    font-family: charter, Georgia, serif;

You can get a convenient copy of Charter here: https://practicaltypography.com/charter.html

Another free font based on (and largely identical to?) Charter is Charis: https://software.sil.org/charis/

[generuso]

Indeed. My mistake.

[tylerneylon]

Thank you!