Hello Michal,
> We will need to make some changes to TeX4ht sources to support the metadata
> block used before \documentclass, because TeX4ht is inactive at that moment,
> so it doesn't track loaded files. It caused problems with my tests, namely
> because it loaded the Amsmath package. There can be potentially much more
> problems.
while there certainly can/will be problems, this error here seems to be
unrelated to \DocumentMetadata. I get an error with make4ht also for this:
\documentclass{article}
\AddToHook{begindocument/before}{\RequirePackage{amsmath}}
\begin{document}
abc
\end{document}
Ulrike
