michal updated tex4ht (r435) and i updated tl (r48958).
hope it flies ...

Posted at

https://tex.stackexchange.com/questions/455994/tex4ht-fails-with-scrartcl-class-when-defining-rm-command-argument-of-popthre

This MWE compiles OK with lualatex and pdflatex but fails with tex4ht. Why? And this happens only when using scrartcl and not with article.

---------------------
\documentclass[11pt]{scrartcl}%{article}
\DeclareOldFontCommand{\rm}{\normalfont\rmfamily}{\mathrm}

\begin{document}
test
\end{document}
------------------

Now

------------------
make4ht -ulm draft foo.tex
....
(/usr/local/texlive/2018/texmf-dist/tex/generic/tex4ht/html5.4ht))
(/usr/local/texlive/2018/texmf-dist/tex/generic/tex4ht/scrartcl.4ht
! Argument of \popthree has an extra }.
<inserted text>
\par
l.33 \:temp{rm}
?
------------------------

I also tried

\documentclass[11pt,enabledeprecatedfontcommands]{scrartcl}

But make4ht still gave same error.