I love the font. Maybe particularly because it's kind of big, so you can see the details. It seems to be EB Garamond: https://en.wikipedia.org/wiki/EB_Garamond
I use it on sonnet.io, consulting.sonnet.io and sit.sonnet.io I didn't use it on days.sonnet.io only because I felt weirdly obsessed with performance when I was coding it.