\makeatletter

\def\HCode{\futurelet\HCode\HChar}\def\HChar{\ifx"\HCode\def\HCode"##1"{\Link##1}\expandafter\HCode\else\expandafter\Link\fi}\def\Link#1.a.b.c.{\g@addto@macro\@documentclasshook{\RequirePackage[#1,html]{tex4ht}}\let\HCode\documentstyle\def\documentstyle{\let\documentstyle\HCode\expandafter\def\csname tex4ht\endcsname{#1,html}\def\HCode####1{\documentstyle[tex4ht,}\@ifnextchar[{\HCode}{\documentstyle[tex4ht]}}}

\makeatother

\HCode "html,uni-html4,png,charset=utf-8".a.b.c.


\documentclass[11pt,a4paper]{book}


\begin{document}

\chapter{UTF-8}

The following characters should be converted to Unicode:

Spanish: áéíóúÁÉÍÓÚ
German: äöüÄÖÜ
Danish: æøåÆØÅ

\end{document}
 
