Comment by ktpsns

The main workhorse is https://dlmf.nist.gov/LaTeXML/ as you can find once you reach https://github.com/arxiv-vanity/engrafo.

LaTeXML converts tex to XML by running latex ("only latex can parse latex") and working on the DVI output.

But nevertheless this is a hard job so I will loook into the engrafo code soon because I want to apply this to a book we have written.