At a high level they use plastex https://github.com/plastex/plastex to convert latex to html (you seem to be using pandoc?) and so can control the rendering to any fine accuracy they want. I liked this general style as well, so I tried using plastex but couldn't get my head around it and so started using LateXML https://github.com/brucemiller/LaTeXML

My usecase: I wanted to have a "dependency graph" of lemmas to make it easier to see proofs without having to jump back and forth through a pdf, and this was sort of similar to lean formalization blueprint graphs https://teorth.github.io/pfr/blueprint/dep_graph_document.ht... (which also uses plastex) but without the lean parts. There's still a lot of work to be done, but I think I have a pretty okay implementation using latexml which meets 50% of my requirements for now, so I'm happyish https://texviz.arsricharan.in/ghrss24/