It's LaTeX, tikz, and pgfplots for the images. Complied with xelatex to pdf and pdf2svg to svg. The rest is custom HTML and CSS. We use the Hakyll static site generator.

Yes, can confirm. Check out [1] and [2] to learn more about TikZ and PGFPlots.

[1]: http://mirrors.ctan.org/graphics/pgf/base/doc/pgfmanual.pdf [2]: http://mirrors.ctan.org/graphics/pgf/contrib/pgfplots/doc/pg...

I particularly appreciate how the text in the diagrams matches the font (& font size) of the body text. Almost nobody does that and it looks great.

The graph looks a lot like GraphViz output. Not sure about the charts, however.

As always, I will bitch about the colour of the text being grey (#333) instead of black. Hurts readability, doesn't improve design.

#333 is a great color for text on the web.

That depends on the medium used to view the web. On my KoboHD e-ink reader, grey text is not legible. It's sad because thousands of web sites that would be gorgeously legible if their text were #000 are not. But this is more of a browser issue; Kobo should disable colors IMO. And this is something that people can do on regular browsers.

objectively its color contrast ratio on a white bg is more than high enough to meet web content accessibility guidelines

Always depends on the color of the background : )

Well, I disagree.