Issue with mhchem and fractions in make4ht and mathml

Question

I am compiling a chemistry-heavy text and I noticed that mhchem on Overleaf did not create superscripts and subscripts when compiled with make4ht. Upon downloading the newest release, which put the support for make4ht back in, it compiled beautifully except for when using fraction bars.

Here is my MWE:

\documentclass{book} 
\usepackage[english]{babel}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage[version=4]{mhchem}

\begin{document}

Text with formula \ce{CuCl2} and \ce{Cu^{2+}}.

\begin{equation}
\frac{\ce{mass CuCl2}}{1}\times\frac{\ce{1 mol CuCl2}}{\ce{MM of CuCl2}}= \mathord{?}\mathord{?}~\ce{ mol CuCl2}
\end{equation}

Text before a reaction.

\begin{equation}
\ce{Al^0 (s) -> Al^3+ (aq) + 3e-}\\
\ce{Cu^2+ (aq) + 2e- -> Cu^0 (s)}
\end{equation}


\end{document}

And I compile that using the following command:

make4ht -f html5 -e mybuild.mk4 -um draft -c mwe.cfg main "early^,early_"

Using this mwe.cfg file:

\Preamble{xhtml,mathml}

\Configure{@HEAD}{\HCode{%
     <script type="text/x-mathjax-config">
       MathJax.Hub.Config({
         extensions: ["tex2jax.js"],
         jax: ["input/TeX", "output/HTML-CSS"],
         tex2jax: {
           \unexpanded{inlineMath: [ ['$','$'], ["\\(","\\)"] ],}
           \unexpanded{displayMath: [ ['$$','$$'], ["\\[","\\]"] ],}
           processEscapes: true
         },
         "HTML-CSS": { availableFonts: ["TeX"] }
       });
     </script>
  }}

\Configure{@HEAD}{\HCode{<script type="text/javascript"\Hnewline
src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"\Hnewline
></script>\Hnewline}}

\Css{.MathJax_MathML {text-indent: 0;}}

\begin{document}
\EndPreamble

The PDF renders fine, but the first equation renders in-line with no fractions.

Answer 1

The problem is that there is no MathML support for Mhchem at the moment, so you get this wrong rendering as a result. I will try to fix it, but until it is done, you can try the following configuration file. It uses built-in MathJax support in TeX4ht, and configures the \ce command to stay in the generated HTML, so it can be rendered by MathJax too:

\Preamble{xhtml,mathjax}
\def\ce#1{\texttt{\detokenize{\(\ce{#1}\)}}}
\begin{document}
\EndPreamble

This is the result:

Alternatively, here is a version of mhchem.4ht with basic MathML support:

%% mhchem.4ht
%% 2021-12-31
%% Copyright 2015-2021 Martin Hensel
%
%% This file is part of the mhchem bundle for LaTeX
%
% This work may be distributed and/or modified under the
% conditions of the LaTeX Project Public License version 1.3c
% which is included as lppl-1-3c.txt.
%
% This work has the LPPL maintenance status "maintained".
% The Current Maintainer of this work is Martin Hensel.
%
% ( In order to fight spam, the maintainer's contact      )
% ( information is "encrypted" with ROT13.                )
% ( If you do not know ROT13 yet and have no tool for     )
% ( decryption, simply do an Internet search for "ROT13". )
%
% ,---[ ROT 13 ]---
% | Gur Pheerag Znvagnvare bs guvf jbex vf Znegva Urafry
% |   jub pna or pbagnpgrq ivn
% |     [email protected]
% |   be ivn znvy
% |     Znegva Urafry
% |     Cbfgfge. 20
% |     09232 Unegznaafqbes
% |     Treznal
% `----------
%
% This work consists of all files listed in manifest.txt.
%
%
\ExplSyntaxOn

\Css
  { 
    .mhchem ~ .mhchem-cf { white-space: nowrap; }
    .mhchem ~ .mhchem-isotope { font-size: 0.65em; line-height: 1; display: inline-block; vertical-align: sub; text-align: right; }
    .mhchem ~ .mhchem-supsub  { font-size: 0.65em; line-height: 1; display: inline-block; vertical-align: sub; }
    .mhchem ~ .mhchem-sup     { font-size: 0.65em; line-height: 1; display: inline-block; vertical-align: 68\%; }
    .mhchem ~ .mhchem-sub     { font-size: 0.65em; line-height: 1; display: inline-block; vertical-align: sub; }
    .mhchem ~ .mhchem-rlap { display: inline-block; width: 0; }
    .mhchem ~ .mhchem-bond-half-top    { vertical-align: 60\%; }
    .mhchem ~ .mhchem-bond-half-middle { vertical-align: 35\%; }
    .mhchem ~ .mhchem-bond-top         { vertical-align: 20\%; }
    .mhchem ~ .mhchem-bond-bottom      { vertical-align: -20\%; }
    .mhchem ~ .mhchem-arrow { display: inline-flex; flex-direction: column; vertical-align: middle; }
    .mhchem ~ .mhchem-arrow > span { line-height: 80\%; text-align: center; }
    .mhchem ~ .mhchem-arrow > .mhchem-above { min-height: 1em; font-size: 60\%; }
    .mhchem ~ .mhchem-arrow > .mhchem-below { min-height: 1em; font-size: 60\%; }
    .mhchem ~ .mhchem-arrow > span.mhchem-arrow-normal { line-height: 40\%; }
    .mhchem ~ .mhchem-arrow > span.mhchem-arrow-larger { line-height: 25\%; font-size: 150\%; }
    .mhchem ~ .mhchem-arrow > span.mhchem-arrow-complement { line-height: 25\%; }
  }

% basic mhchem containers
\NewConfigure{mhchemce}{2}
\NewConfigure{mhchemcf}{2}

\pend:def\__mhchem_output_begin_ce:{\a:mhchemce}
\append:def\__mhchem_output_end_ce:{\b:mhchemce}
\pend:def\__mhchem_output_begin_cf:{\a:mhchemcf}
\append:def\__mhchem_output_end_cf:{\b:mhchemcf}

\Configure{mhchemcf}{\HCode{<span~class="mhchem">}}{\HCode{</span>}}
\Configure{mhchemce}{\HCode{<span~class="mhchem">}}{\HCode{</span>}}


% alternative version of coreFivd

\NewConfigure{mhchemisotope}{3}
\Configure{mhchemisotope}{\HCode{<span~class="mhchem-isotope">}}{\HCode{<br~/>&\#xfeff;}}{\HCode{</span>}}

\NewConfigure{mhchemsub}{2}
\Configure{mhchemsub}{\HCode { <sub~class="mhchem-sub"> }}{\HCode { </sub> } }

\NewConfigure{mhchemsup}{2}
\Configure{mhchemsup}{\HCode { <sup~class="mhchem-sup"> }}{\HCode { </sup> } }

\cs_set_protected:Npn \__mhchem_output_coreFivefourht:nnnnnnn #1#2#3#4#5#6#7
  {
    \bool_if:nT
      { \tl_if_empty_p:n {#7}  &&  ! \tl_if_empty_p:n {#5} }
      { \bool_set_true:N \l__mhchem_option_superscriptsStacked_bool }
      
    \tl_if_empty:nF {#1#2} 
      {
        \a:mhchemisotope
        \__mhchem_output_withFont:n { #1 } 
        \b:mhchemisotope
        \__mhchem_output_withFont:n { #2 }
        \c:mhchemisotope
      }
    \__mhchem_output_withFont:n { #3 } 
    \bool_if:NTF \l__mhchem_option_superscriptsStacked_bool
      {
        \tl_if_empty:nTF {#4#5#7} 
          {
            \tl_if_empty:nF {#6}
              {
                \a:mhchemsub
                \__mhchem_output_withFont:n { #6 }
                \b:mhchemsub
              }
          }
          {
            \tl_if_empty:nTF {#6}
              {
                \a:mhchemsup
                \__mhchem_output_withFont:n { #4#5#7 }
                \b:mhchemsup
              }
              { 
                \HCode { <span~class="mhchem-supsub"> }
                \__mhchem_output_withFont:n { #4#5#7 }
                \HCode { <br~/>&\#xfeff; }
                \__mhchem_output_withFont:n { #6 }
                \HCode { </span> } 
             }
          }
      }
      {
        \tl_if_empty:nF {#4}
          {
            \a:mhchemsup
            \__mhchem_output_withFont:n { #4 }
            \b:mhchemsup
          }
        \tl_if_empty:nF {#6}
          { 
            \a:mhchemsub
            \__mhchem_output_withFont:n { #6 }
            \b:mhchemsub
          }
        \tl_if_empty:nF {#5#7} 
          { 
            \a:mhchemsup
            \__mhchem_output_withFont:n { #5#7 }
            \b:mhchemsup
          }
      }
  }

% replace original coreFive with our alternative version. it will work in picture math
\HLet\__mhchem_output_coreFive:nnnnnnn\__mhchem_output_coreFivefourht:nnnnnnn

\cs_set_protected:Npn \__mhchem_output_withFont:n #1  %. output #1 as math or text
  {
    \bool_if:NTF \l__mhchem_output_isMathMode_bool
      { \mathrm {#1} }
      { \text {#1} }
  }

\NewConfigure{mhchemoperator}{2}
\Configure{mhchemoperator}{\HCode { <span~class="mhchem-operator"> ~ }}{\HCode { ~ </span> }}

% ideally, these should be redefined using \HLet, to support picture math
\cs_set_protected:Npn \__mhchem_output_skipAfterAmount: { \HCode { ~ } }  % space for copy & paste
\cs_set_protected:Npn \__mhchem_output_skipBeforeStateOfAggregation: {}
\cs_set_protected:Npn \__mhchem_output_minus: { \HCode { &minus; } }
\cs_set_protected:Npn \__mhchem_output_operatorPlus:
  { \a:mhchemoperator + \b:mhchemoperator  }
\cs_set_protected:Npn \__mhchem_output_operatorMinus:
  { \a:mhchemoperator\HCode {&minus;} \b:mhchemoperator }
\cs_set_protected:Npn \__mhchem_output_operatorEquals:
  { \a:mhchemoperator =  \b:mhchemoperator }
\cs_set_protected:Npn \__mhchem_output_operatorPlusMinus:
  {\a:mhchemoperator \HCode {&plusmn; } \b:mhchemoperator }
\cs_set_protected:Npn \__mhchem_output_electronDot: { \HCode { &bull; } }
\cs_set_protected:Npn \__mhchem_output_additionCompound: { \HCode { &middot; } }
\cs_set_protected:Npn \__mhchem_output_excited: { \HCode { &\#x2731; } }
\cs_set_protected:Npn \__mhchem_output_commaDecimal: { , }
\cs_set_protected:Npn \__mhchem_output_commaEnumeration: { , \HCode { &thinsp; } }
\cs_set_protected:Npn \__mhchem_output_commaEnumerationSmall: { , \HCode { &thinsp; } }
  
\cs_set_protected:Npn \__mhchem_output_bond_single: { \HCode { &minus; } }
\cs_set_protected:Npn \__mhchem_output_bond_double: { \HCode { &equals; } }
\cs_set_protected:Npn \__mhchem_output_bond_triple: { \HCode { &equiv; } }
% todo: add MathML support for these
\cs_set_protected:Npn \__mhchem_output_bond_half:  % todo: improve
  { \HCode { <span~class="mhchem-bond-half-middle">&\#xfe4d;</span> } }
\cs_set_protected:Npn \__mhchem_output_bond_oneAndHalf:  % todo: improve 
  { \HCode { <span~class="mhchem-rlap">&minus;</span><span~class="mhchem-bond-half-top">&\#xfe4d;</span> } }
\cs_set_protected:Npn \__mhchem_output_bond_twoAndHalf:  % todo: improve
  { \HCode { <span~class="mhchem-rlap">=</span><span~class="mhchem-bond-half-top">&\#xfe4d;</span> } }
\cs_set_protected:Npn \__mhchem_output_bond_twoAndHalff:  % todo: improve
  { \HCode { <span~class="mhchem-rlap~mhchem-bond-top">&minus;</span><span~class="mhchem-rlap~mhchem-bond-bottom">&minus;</span><span~class="mhchem-bond-half-middle">&\#xfe4d;</span> } }
\cs_set_protected:Npn \__mhchem_output_bond_dotdotdot: { \HCode { &middot;&middot;&middot; } }
\cs_set_protected:Npn \__mhchem_output_bond_dotdotdotdot: { \HCode { &middot;&middot;&middot;&middot; } }
\cs_set_protected:Npn \__mhchem_output_bond_rightArrow: { \HCode { &\#xffeb; } }
\cs_set_protected:Npn \__mhchem_output_bond_leftArrow: { \HCode { &\#xffe9; } }

\NewConfigure{mhchemarrow}{2}
\NewConfigure{mhchemarrowabove}{2}

\NewConfigure{mhchemarrowyields}{1}
\NewConfigure{mhchemarrowyieldsLeft}{1}
\NewConfigure{mhchemarrowyieldsLeftRight}{1}
\NewConfigure{mhchemarrowmesomerism}{1}
\NewConfigure{mhchemarrowequilibrium}{1}
\NewConfigure{mhchemarrowequilibriumRight}{1}
\NewConfigure{mhchemarrowequilibriumLeft}{1}

\Configure{mhchemarrow}{\HCode {<span~class="mhchem-arrow">}}{\HCode{</span>}}
\Configure{mhchemarrowabove}{\HCode { <span~class="mhchem-above">}}{\HCode{</span>}}
\Configure{mhchemarrowyields}{ \HCode { <span~class="mhchem-arrow-normal">&\#x27F6;</span> } }
\Configure{mhchemarrowyieldsLeft}{ \HCode { <span~class="mhchem-arrow-normal">&\#x2190;</span> } }
\Configure{mhchemarrowyieldsLeftRight}{ \HCode { <span~class="mhchem-arrow-normal">&\#x21c4;</span> } }  % todo: improve
\Configure{mhchemarrowmesomerism}{ \HCode { <span~class="mhchem-arrow-normal">&\#x2194;</span> } }  % todo: improve
\Configure{mhchemarrowequilibrium}{ \HCode { <span~class="mhchem-arrow-normal">&\#x21cc;</span> } }  % todo: improve
\Configure{mhchemarrowequilibriumRight}{ \HCode { <span~class="mhchem-arrow-larger">&\#x21c0;</span><span~class="mhchem-arrow-complement">&\#x21bd;</span> } }  % todo: improve
\Configure{mhchemarrowequilibriumLeft}{ \HCode { <span~class="mhchem-arrow-complement">&\#x21c0;</span><span~class="mhchem-arrow-larger">&\#x21bd;</span> } }  % todo: improve



\cs_set_protected:Npn \__mhchem_arrow_deployfourht:nnnnn #1#2#3#4#5
  {
    \a:mhchemarrow
    \tl_if_empty:nF {#3#5}
      {
        \a:mhchemarrowabove
        \str_case:nnF {#2}
          {
            {   } { \ce { #3 } }
            { M } { \ensuremath { #3 } }
            { T } { \text { #3 } }
            { C } { \ce { #3 } }
          }
          { \msg_error:nnn { mhchem } { unexpected-arrow-type } {#2} }
          \b:mhchemarrowabove
      }
    \str_case:nnF {#1}
      {
        { yields } {\a:mhchemarrowyields }
        { yieldsLeft } {\a:mhchemarrowyieldsLeft }
        { yieldsLeftRight } {\a:mhchemarrowyieldsLeftRight }  % todo: improve
        { esomerism } {\a:mhchemarrowesomerism }  % todo: improve
        { equilibrium } {\a:mhchemarrowequilibrium }  % todo: improve
        { equilibriumRight } {\a:mhchemarrowequilibriumRight }  % todo: improve
        { equilibriumLeft } {\a:mhchemarrowequilibriumLeft }  % todo: improve
      }
      { \msg_error:nnn { mhchem } { unexpected-arrow-type } {#1} }
    \tl_if_empty:nF {#3#5}
      {
        \a:mhchemarrowabove
        \str_case:nnF {#2}
          {
            {   } { \ce { #5 } }
            { M } { \ensuremath { #5 } }
            { T } { \text { #5 } }
            { C } { \ce { #5 } }
          }
          { \msg_error:nnn { mhchem } { unexpected-arrow-type } {#2} }
        \b:mhchemarrowabove
      }
    \b:mhchemarrow
  }

\HLet\__mhchem_arrow_deploy:nnnnn\__mhchem_arrow_deployfourht:nnnnn
  
\ExplSyntaxOff

% Additional configurations for MathML

% print mathml command or html, depending on if we are inside mathml or not
\newcommand\:mhmathmlorhtml[2]{\ifmathml\a:mathml #1\else #2\fi}
\newcommand\:mhonlyinmathml[1]{\ifmathml #1\fi}
\:CheckOption{mathml}
\if:Option

\Configure{mhchemcf}{\HCode{<\:mhmathmlorhtml{mrow}{span}\space class="mhchem-cf">}}{\HCode{</\:mhmathmlorhtml{mrow}{span}>}}
\Configure{mhchemce}{\HCode{<\:mhmathmlorhtml{mrow}{span}\space class="mhchem-ce">}}{\HCode{</\:mhmathmlorhtml{mrow}{span}>}}

\Configure{mhchemsub}{%
  \:mhmathmlorhtml{%
  \Send{BACK}%
  {<\a:mathml msub><\a:mathml mrow\Hnewline>}%
  \Tg</\a:mathml mrow>\Tg<\a:mathml mrow\Hnewline>\trap:base%
  }{%
  \HCode{<sub>}}%
}{\:mhmathmlorhtml{\Tg</\a:mathml mrow>\TG</\a:mathml msub\Hnewline>}{\HCode{</sub>}}}
\Configure{mhchemsup}{%
  \:mhmathmlorhtml{
  \Send{BACK}
  {<\a:mathml msup><\a:mathml mrow\Hnewline>}%
  \Tg</\a:mathml mrow>\Tg<\a:mathml mrow\Hnewline>\trap:base
  }{%
  \HCode{<sup>}}%
}{\:mhmathmlorhtml{\Tg</\a:mathml mrow>\TG</\a:mathml msup\Hnewline>}{\HCode{</sup>}}}

\Configure{mhchemoperator}{\HCode{<\:mhmathmlorhtml{mo}{span} class="mhchem-operator">}\PauseMathClass}{\EndPauseMathClass\HCode {</\:mhmathmlorhtml{mo}{span}>}}

\Configure{mhchemarrow}{\HCode {<\:mhmathmlorhtml{mrow}{span} class="mhchem-arrow">\:mhonlyinmathml{<mo>}}}{\HCode{\:mhonlyinmathml{</mo>}</\:mhmathmlorhtml{mrow}{span}>}}
\Configure{mhchemarrowabove}{\HCode { <\:mhmathmlorhtml{mrow}{span} class="mhchem-above">\:mhonlyinmathml{<mo>}}}{\HCode{\:mhonlyinmathml{</mo>}</\:mhmathmlorhtml{mrow}{span}>}}
\Configure{mhchemarrowyields}{ \HCode { <\:mhmathmlorhtml{mrow}{span} class="mhchem-arrow-normal">\:mhonlyinmathml{<mo>}&\#x2192;\:mhonlyinmathml{</mo>}</\:mhmathmlorhtml{mrow}{span}> } }
\Configure{mhchemarrowyieldsLeft}{ \HCode { <\:mhmathmlorhtml{mrow}{span} class="mhchem-arrow-normal">\:mhonlyinmathml{<mo>}&\#x2190;\:mhonlyinmathml{</mo>}</\:mhmathmlorhtml{mrow}{span}> } }
\Configure{mhchemarrowyieldsLeftRight}{ \HCode { <\:mhmathmlorhtml{mrow}{span} class="mhchem-arrow-normal">\:mhonlyinmathml{<mo>}&\#x21c4;\:mhonlyinmathml{</mo>}</\:mhmathmlorhtml{mrow}{span}> } }  % todo: improve
\Configure{mhchemarrowmesomerism}{ \HCode { <\:mhmathmlorhtml{mrow}{span} class="mhchem-arrow-normal">\:mhonlyinmathml{<mo>}&\#x2194;\:mhonlyinmathml{</mo>}</\:mhmathmlorhtml{mrow}{span}> } }  % todo: improve
\Configure{mhchemarrowequilibrium}{ \HCode { <\:mhmathmlorhtml{mrow}{span} class="mhchem-arrow-normal">\:mhonlyinmathml{<mo>}&\#x21cc;\:mhonlyinmathml{</mo>}</\:mhmathmlorhtml{mrow}{span}> } }  % todo: improve
\Configure{mhchemarrowequilibriumRight}{ \HCode { <\:mhmathmlorhtml{mrow}{span} class="mhchem-arrow-larger">\:mhonlyinmathml{<mo>}&\#x21c0;\:mhonlyinmathml{</mo>}</\:mhmathmlorhtml{mrow}{span}><\:mhmathmlorhtml{mrow}{span} class="mhchem-arrow-complement">\:mhonlyinmathml{<mo>}&\#x21bd;\:mhonlyinmathml{</mo>}</\:mhmathmlorhtml{mrow}{span}> } }  % todo: improve
\Configure{mhchemarrowequilibriumLeft}{ \HCode { <\:mhmathmlorhtml{mrow}{span} class="mhchem-arrow-complement">\:mhonlyinmathml{<mo>}&\#x21c0;\:mhonlyinmathml{</mo>}</\:mhmathmlorhtml{mrow}{span}><\:mhmathmlorhtml{mrow}{span} class="mhchem-arrow-larger">\:mhonlyinmathml{<mo>}&\#x21bd;\:mhonlyinmathml{</mo>}</\:mhmathmlorhtml{mrow}{span}> } }  % todo: improve
\else
\fi
% }

\Hinput{mhchem}

The version of this file that is included in TeX Live mixes command redefinitions with the HTML code. It is better not to mix them but to use configurable hooks instead. In this way, you can support multiple output formats.

You can save this file to the same directory as your TeX file, it will be used automatically.

Here is a result with

 make4ht filename.tex "mathml,mathjax"

You need up-to-date version of TeX4ht, and development version of make4ht to get it work.