Question: there is htmex.bat but I don't see htmex anywhere in the
mk4ht.pl.
I don't either.
Can it be mapped to:
mk4ht ht mex ...
Looking at the scripts on Unix, it seems likely. I don't know.
Try it ...
What is this mex anyway?
A format invented by the Polish guys long long ago. There is a "mex"
executable + support files in TL.