documentclass[11pt]{scrartcl}%{article}
DeclareOldFontCommand{rm}{normalfontrmfamily}{mathrm}

begin{document}
test
end{document}

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}
?

documentclass[11pt,enabledeprecatedfontcommands]{scrartcl}