As an arXiv author who likes using complicated TeX constructions, the introduction of HTML conversion has increased my workload a lot trying to write fallback macros that render okay after conversion. The conversion is super slow and there is no way to faithfully simulate it locally. Still I think it's a great thing to do.

I believe dginev's Docker image https://github.com/dginev/ar5ivist is very close to what runs on arXiv and can be run locally. It uses a recent LaTeXML snapshot from September.