These functions are represented as LaTeX, because that's the gold standard for rendering math. But, they also want them available as XML so they're in a more sensible format for archiving.

So, funnily, as part of this, NIST maintain a LaTeX to XML converter.[0] It's a pretty complete LaTeX runtime. (Yes, runtime -- TeX is a programming language, not a markup language!)

This is the thing that powers arXiv Vanity[1] and other academic publishing projects.

I just find it quite amusing that the US Government maintains a LaTeX runtime written in Perl.

[0] https://dlmf.nist.gov/LaTeXML/

[1] https://www.arxiv-vanity.com/