committed to tl (some time ago, i believe), so closing.

reference:

https://tex.stackexchange.com/questions/511350/why-adding-pagestylefancy-gives-compile-error-with-tex4ht

This MWE

---------------
\documentclass[12pt]{article}%
\usepackage{fancyhdr}
\pagestyle{fancy}

\begin{document}
\section{$\cos \left(x\right)$}
test
\end{document}
-----------------

Gives this error

------------
make4ht foo1.tex
........
(/usr/local/texlive/2019/texmf-dist/tex/generic/tex4ht/html4-math.4ht)
(/usr/local/texlive/2019/texmf-dist/tex/generic/tex4ht/html5.4ht)) (./foo1.aux)
! Argument of \im:g has an extra }.
<inserted text>
\par
l.6 \section{$\cos \left(x\right)$}

?
-----------------

commenting out \pagestyle{fancy} the error goes away.

Also, which is more interesting, changing \section{$\cos \left(x\right)$} to \section{$\cos x$} the error goes away! i.e.

----------------
\documentclass[12pt]{article}%
\usepackage{fancyhdr}
\pagestyle{fancy}

\begin{document}
\section{$\cos x$}
test
\end{document}
----------------

No error. So it looks like it is the use of \left and \right is what caused the problem for text4ht when using fancyhrd

TL 2019 with latest updates as of 2 days ago.