why adding \pagestyle{fancy} gives compile error with tex4ht?

Question

This MWE

\documentclass[12pt]{article}%
\usepackage{fancyhdr}
\pagestyle{fancy}

\begin{document}
\section{$\cos \left(x\right)$}
test
\end{document}

Gives this error

make4ht foo1.tex
........
(/usr/local/texlive/2019/texmf-dist/tex/generic/tex4ht/html4-math.4ht)
(/usr/local/texlive/2019/texmf-dist/tex/generic/tex4ht/html5.4ht)) (./foo1.aux)
! Argument of \im:g has an extra }.
<inserted text>
                \par
l.6 \section{$\cos \left(x\right)$}

?

commenting out \pagestyle{fancy} the error goes away.

Also, which is more interesting, changing \section{$\cos \left(x\right)$} to \section{$\cos x$} the error goes away! i.e.

\documentclass[12pt]{article}%
\usepackage{fancyhdr}
\pagestyle{fancy}

\begin{document}
\section{$\cos x$}
test
\end{document}

No error. So it looks like it is the use of \left and \right is what caused the problem for text4ht when using fancyhrd

I will change my code so to avoid loading fancyhdr when in tex4ht mode to bypass this.

But the question is, why make4ht fails like this? Should make4ht handle this better?

TL 2019 with latest updates as of 2 days ago.

Answer 1

Here is definition of the fancy page style:

\def\ps@fancy{%
  \f@nch@ifundefined{@chapapp}{\let\@chapapp\chaptername}{}% for amsbook
\f@nch@ifundefined{MakeUppercase}{\def\MakeUppercase{\uppercase}}{}%
\f@nch@ifundefined{chapter}%
 {\def\sectionmark##1{\markboth{\MakeUppercase{\ifnum \c@secnumdepth>\z@
        \thesection\hskip 1em\relax
      \fi ##1}}{}}%
  \def\subsectionmark##1{\markright {\ifnum \c@secnumdepth >\@ne
    \thesubsection\hskip 1em\relax \fi ##1}}}%
 {\def\chaptermark##1{\markboth {\MakeUppercase{\ifnum
      \c@secnumdepth>\m@ne \@chapapp\ \thechapter. \ \fi ##1}}{}}%
\def\sectionmark##1{\markright{\MakeUppercase{\ifnum \c@secnumdepth >\z@
      \thesection. \ \fi ##1}}}%
}
\ps@@fancy
\gdef\ps@fancy{\@fancyplainfalse\ps@@fancy}%
\ifdim\headwidth<0sp
    \global\advance\headwidth123456789sp\global\advance\headwidth\textwidth
\fi}

As you can see, it redefines \sectionmark and other section related commands. It seems that it clashes with tex4ht. As the functionality provided by fancyhdr doesn't make sense for the HTML output, I think it is best to just redefine the \ps@fancy command to do nothing. The only catch is that because the \pagestyle command can be used in the document preamble, the patch cannot be incorporated using the fancyhdr.4ht file. We must use the tex4ht mechanism for patching packages in \usepackage. Here is the full updated `usepackage.4ht:

% usepackage.4ht (2019-06-24-13:58), generated from tex4ht-4ht.tex
% Copyright 2003-2009 Eitan M. Gurari
% Copyright 2009-2019 TeX Users Group
%
% This work may be distributed and/or modified under the
% conditions of the LaTeX Project Public License, either
% version 1.3c of this license or (at your option) any
% later version. The latest version of this license is in
%   http://www.latex-project.org/lppl.txt
% and version 1.3c or later is part of all distributions
% of LaTeX version 2005/12/01 or later.
%
% This work has the LPPL maintenance status "maintained".
%
% The Current Maintainer of this work
% is the TeX4ht Project <http://tug.org/tex4ht>.
%
% If you modify this program, changing the
% version identification would be appreciated.
\immediate\write-1{version 2019-06-24-13:58}

   \def\:temp{tex4ht}\ifx \:temp\@currname
   \:warning{\string\usepackage{tex4ht} again?}
   \def\:temp#1htex4ht.def,tex4ht.sty#2!*?: {\def\:temp{#2}}
\expandafter\:temp \@filelist htex4ht.def,tex4ht.sty!*?: %
\ifx \:temp\empty  \else
   \:warning{if
    \string\RequirePackage[tex4ht]{hyperref} or
    \string\usepackage[tex4ht]{hyperref} was
    used try instead, repectively,
    \string\RequirePackage{hyperref} or
    \string\usepackage{hyperref}}
\fi

\fi
\gdef\a:usepackage{\use:package xr,xr-hyper,eso-pic,expl3,savetrees,biblatex,fancyhdr,xeCJK,polyglossia,fontspec,tikz,pdfbase,graphics,xcolor,imakeidx,minted,,!*?: }
\gdef\use:package#1,{%
   \if :#1:\def\:temp##1!*?: {}\else
      \def\:temp{#1}\ifx \@currname\:temp
             \def\:temp##1!*?: {\input usepackage.4ht  }%
      \else \let\:temp=\use:package \fi
   \fi \:temp}
\def\:temp{xr}\ifx \@currname\:temp
   \AtEndOfPackage{\let\XR:\XR@
   \def\XR@[#1]#2{%
    \Configure{AtBeginDocument}{\XR:[#1]{#2}}{}}%
}

\fi
\def\:temp{xr-hyper}\ifx \@currname\:temp
   \def\eat:optional[#1]{}
\AtEndOfPackage{\let\XR:\XR@
   \def\XR@[#1]#2{%
     \Configure{AtBeginDocument}{\XR:[#1]{#2}}{}\@ifnextchar[\eat:optional{}}%
}

\fi
\def\:temp{eso-pic}\ifx \@currname\:temp
\:dontusepackage{eso-pic}
\newcommand\AddToShipoutPicture{\@ifstar{}{}}
\let\AddToShipoutPictureBG\AddToShipoutPicture
\let\AddToShipoutPictureFG\AddToShipoutPicture
\fi
\def\:temp{expl3}\ifx \@currname\:temp
\ifdefined\XeTeXversion%
\xenunidelblock{Latin-expl3}%
\AtEndOfPackage{\xeuniuseblock{Latin-expl3}}
\fi
\fi
\def\:temp{savetrees}\ifx \@currname\:temp
\:dontusepackage{savetrees}
\fi
\def\:temp{biblatex}\ifx \@currname\:temp
\AtEndOfPackage{%
  \def\blx@mknoautolang{%
    \blx@lbxinput{\blx@languagename}%
    {}{}%
  }%
  \def\blx@lbxinput@iii#1#2{%
  \global\csundef{blx@lng@#2}%
  \:IfFileExists{#1.lbx}
    {\blx@lbxinput@iv{#2}{#1}{language '#2' -> '#1'}}
    {\ifcsdef{blx@suffmaptried@#2}
      {}
      {\blx@warning@noline{%
          File '#1.lbx' not found!\MessageBreak
          Ignoring mapping '#2' -> '#1'}%
       \global\cslet{blx@suffmaptried@#2}\@empty}%
     \blx@lbxinput@iv{#2}{#2}{language '#2'}}}

}
\fi
\def\:temp{xeCJK}\ifx \@currname\:temp
\:dontusepackage{xeCJK}
\AtEndOfPackage{%
  \RequirePackage{fontspec}
}
\DeclareDocumentCommand\setCJKmainfont{o m o}{}
\let\setCJKsansfont\setCJKmainfont
\let\setCJKmonofont\setCJKmainfont

\DeclareDocumentCommand\setCJKfamilyfont {m o m }{}
\DeclareDocumentCommand\newCJKfontfamily {o m o m}{\expandafter\gdef\csname #2\endcsname{\relax}}

\DeclareDocumentCommand\xeCJKsetup{m}{}
% }
\AtBeginDocument{%
  \xeuniuseblock{CJK}
}
\fi
\AtEndOfPackage{
  \let\orig@xpg@define@keys\xpg@define@keys
  \def\xpg@define@keys#1{%
    \orig@xpg@define@keys{#1}%
    \define@choicekey+[xpg@setup]{#1}{direction}[\val\nr]{LR,RL}[LR]%
    {
      \togglefalse{#1@RL}%
      \ifcase\nr\relax%
        \csgappto{init@extras@#1}{%
          a:PolyglossiaRtl%
        }
      \or
        \csgappto{init@extras@#1}{%
          \b:PolyglossiaRtl%
        }
      \fi
    }%
    {\xpg@warning{#1: Invalid value \val\space for key "direction"}}%
}}

\def\:temp{fontspec}\ifx \@currname\:temp
  \ifdefined\fontspec
    \:dontusepackage{fontspec}
  \else
    \input usepackage-fontspec.4ht
  \fi
\fi
\def\:temp{tikz}\ifx \@currname\:temp
\AtEndOfPackage{%
\let\use:tikzlibrary\usetikzlibrary
\def\find:externalize#1external#2\@nil{%
\if\relax#2\relax\else
  \let\tikz:externalize\tikzexternalize
  \renewcommand\tikzexternalize[1][]{\tikz:externalize[##1,mode=only graphics]}
    \tikzset{%
      tex4ht inc/.style={%
        /pgf/images/include external/.code={%
          \includegraphics[]{####1.pdf}%
        }%
      }
    }
    \tikzset{tex4ht inc}
  \fi
}
\renewcommand\usetikzlibrary[1]{%
  \use:tikzlibrary{#1}%
  \find:externalize#1external\@nil%
}
}
\fi
\def\:temp{pdfbase}\ifx\@currname\:temp
\AtEndOfPackage{%
  \let\old:outputpage\@outputpage
  \def\@outputpage{\old:outputpage{}}
}
\fi
\def\:temp{graphics}\ifx\@currname\:temp
  \ifdefined\XeTeXversion
    \PassOptionsToPackage{dvips}{graphics}
  \fi
\fi
\def\:temp{xcolor}\ifx\@currname\:temp
  \ifdefined\XeTeXversion
    \PassOptionsToPackage{dvips}{xcolor}
  \fi
\fi
\def\:temp{imakeidx}\ifx\@currname\:temp
  \PassOptionsToPackage{noautomatic}{imakeidx}
\fi
\def\:temp{minted}\ifx\@currname\:temp
  \AtEndOfPackage{%
    \define@booleankey{minted@opt@g}{breaklines}{}{}{}%
  }
\fi

\def\:temp{fancyhdr}\ifx\@currname\:temp
  \AtEndOfPackage{%
    \def\ps@fancy{}
  }
\fi



\endinput

The part that handles with fancyhdr is small:

\def\:temp{fancyhdr}\ifx\@currname\:temp
  \AtEndOfPackage{%
    \def\ps@fancy{}
  }
\fi

This is the result: