A source file:
\documentclass{book}
\newenvironment{mylist} {\trivlist}{\endtrivlist}
\usepackage[mycfg]{tex4ht}
\begin{document}
\begin{mylist} \item First item
\item Second item
\end{mylist}
\end{document}
and a possible configuration file `mycfg.cfg' for the source file:
\Preamble{html}
\ConfigureList{mylist}
{\HCode{<div class="mylist">}} {\HCode{</div>}} {* }{}
\Css{.mylist { color : red; }}
% see cascade style sheets for
% the meaning of \Css
\begin{document}
\HCode{<!--created by me-->}
\EndPreamble