I googled to see if this was mentioned before, but did not. So making a record for it.

Package paracol does not work in tex4ht. No error, but it does nothing in the output. i.e. does not split the output into columns.

MWE

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\documentclass[11pt]{book}%ext
\usepackage[margin=1in]{geometry}
\usepackage{paracol, blindtext, setspace}
\setlength{\columnseprule}{.4pt}
\setlength{\columnsep}{20pt}
\renewcommand{\baselinestretch}{1.5}\normalsize
\usepackage{blindtext}

\begin{document}
\begin{paracol}{2}
\blindtext
\switchcolumn
\blindtext
\end{paracol}
\end{document}
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compiled with htlatex foo.tex