I love this -- I'll have to do something like that for my site. I always liked the big initials on the start of a paragraph. Though it feels a bit more prose-applicable than for non-fiction writing.
Technically "it depends on the browser settings," but the body font Alegreya is served directly by the site, so I think it would be the one used in almost all cases.
The math fonts used in the formulas are just the ones provided by KaTeX, which I think are just TeX's default math fonts.
--edit: The font used for those initials is called Goudy Initialen: https://www.dafont.com/goudy-initialen.font