Limitations
TeX4ht
doesn’t
understand
features
for
which
it
is
not
configured
\newcommand\myfrac[2]{\vbox{$#1$ \par\vrule\par $#2$}}
\begin{document}
...
\myfrac{#1}{#2}
...
Users may configure features in configuration files
\Preamble{}
\def\myfrac{\frac}
\begin{document}
\EndPreamble
A few features are not understood correctly by TeX4ht
$a {}_1^2b$ % wrong for MathML
They require users’ interface.
$a {\csname HCode\endcsname{}}_1^2b$
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