Another LaTeX-to-HTML tool is lwarp (https://github.com/bdtc/lwarp) which starts from the idea that there only exists one program that can parse LaTeX: the LaTeX compiler itself. Implementing a new parser is almost futile. So instead, the lwarp package redefines all the macros to output HTML. Something like \renewcommand[1]{\textbf}{<b>#1</b>} This way, compiling LaTeX gives you a PDF whose text is HTML code, so now you can extract the plain text from it and you have an HTML file. The advantage is that it can easily deal with custom macros etc., because these are natively resolved by the LaTeX compiler.

I use lwarp to make https://tikz.dev/, an HTML version of the TikZ manual, which is probably one of the most complicated LaTeX documents in existence.