Add warning on future breakage

Michal will update tex4ht ditribution... this will eventually make it to
texlive then to the CI server and it will break.  Adding his comment in
the code so we know.

book/pgfsys-dvisvgm4ht.def

@@ -0,0 +1,75 @@
+% Copyright 2021-2024 by Michal Hoftich
+% Copyright 2006 by Till Tantau
+%
+% This file may be distributed and/or modified
+%
+% 1. under the LaTeX Project Public License and/or
+% 2. under the GNU Public License.
+%
+% See the file doc/generic/pgf/licenses/LICENSE for more details.
+
+\ProvidesFileRCS{pgfsys-dvisvgm4ht.def}
+
+% Driver commands for tex4ht
+
+%
+% Load common pdf commands:
+%
+
+% we switched to dvisvgm driver by default. it supports patterns and other features 
+% dvips driver is available through the tikz+ option. It doesn't support everything, 
+% but it worked better with nested pictures in the past.
+\ifdefined\ifOption
+\ifOption{tikz+}{\input pgfsys-dvips.def}{\input pgfsys-dvisvgm.def}
+\else
+% load the dvips driver by default
+\input pgfsys-dvisvgm.def
+\fi
+
+
+\catcode`\:=11%
+
+% we must call most of these redefinitions in \AtBeginDocument, because \HLet is available 
+% only at that moment
+\AtBeginDocument{%
+ % configure the output picture format to svg, as it will require dvisvgm
+ % post processing. 
+ \Configure{Picture}{.svg}%
+
+ % insert picture hooks to pgfsys commands 
+ % these redefinitions are usually called only with the \tikz command, 
+ % they are ignored in tikzpicture environment
+ \def\:tempa#1{%
+     \texfourht@tikz@begin%
+     \csname o:pgfsys@typesetpicturebox:\endcsname{#1}
+     \texfourht@tikz@end%
+ }
+ \HLet\pgfsys@typesetpicturebox\:tempa
+
+ % we must remove Picture-alt in \pgfsys@beginpicture, because it can result in alt text included in the image
+ \def\:tempa{\Configure{Picture-alt}{}\texfourht@tikz@begin\o:pgfsys@beginpicture:}
+ \HLet\pgfsys@beginpicture\:tempa
+ \let\o:pgfsys@endpicture:\pgfsys@endpicture
+ \def\:tempa{\o:pgfsys@endpicture:}
+ \HLet\pgfsys@endpicture\:tempa
+
+ %  start picture around TikZ and PGF environments
+ \ConfigureEnv{tikzpicture}{\begingroup\texfourht@tikz@begin}{\texfourht@tikz@end\endgroup}{}{}%
+ \ConfigureEnv{pgfpicture}{\begingroup\texfourht@tikz@begin}{\texfourht@tikz@end\endgroup}{}{}%
+}
+
+\def\texfourht@tikz@begin{
+  \protect\csname nested:math\endcsname% support display math
+  \Picture*[\csname a:Picture-alt\endcsname]{}%
+}
+\def\texfourht@tikz@end{\EndPicture}
+
+\catcode`\:=12%
+
+
+\endinput
+
+
+%%% Local Variables:
+%%% mode: latex
+%%% End:

book/tex4ebook.cfg

@@ -19,4 +19,27 @@
     font-family: 'Open Sans';
   }
 </style>}}
+
+% Deal with chemical equations in a single svg
+\ExplSyntaxOn
+\def\standaloneenv#1{}
+\pend:def\schemestart{\Picture+{}}
+\append:def\CF_schemestop{\EndPicture}
+\ExplSyntaxOff
+\makeatletter
+\def\texfourht@tikz@begin{
+  \protect\csname nested:math\endcsname% support display math
+  \Picture+[\csname a:Picture-alt\endcsname]{}%
+}
+\makeatother
+
+% Warnig from Michal: Note that once you get the updated TeX Live with a new
+% `chemfig.4ht` file, you can remove this from the config file:
+%
+%```latex \pend:def\schemestart{\Picture+{}}
+%\append:def\CF_schemestop{\EndPicture} ```
+%
+%You could get an error otherwise, as there would be nested `\Picture+{}` and
+%`\EndPicture` commands.
+
 \EndPreamble

book/website.cfg

@@ -47,5 +47,17 @@
 }
 \ConfigureToc{loc}{\HCode{<span class="lofToc">}}{\ }{}{\HCode{</span><br />}}
 
+\ExplSyntaxOn
+\def\standaloneenv#1{}
+\pend:def\schemestart{\Picture+{}}
+\append:def\CF_schemestop{\EndPicture}
+\ExplSyntaxOff
+\makeatletter
+\def\texfourht@tikz@begin{
+  \protect\csname nested:math\endcsname% support display math
+  \Picture+[\csname a:Picture-alt\endcsname]{}%
+}
+\makeatother
+
 \begin{document}
 \EndPreamble