a template to inject css and js to htlatex or tex4ht
\Preamble{xhtml,mathml,html5}
\Configure{@HEAD}{\HCode{<script></script>\Hnewline}}
\Configure{@HEAD}{\HCode{<style>body {widhth: 40em;}\Hnewline </style>\Hnewline}}
\begin{document}
\EndPreamble
There is a css tag in the third line!!!