I just discovered, that it is possible to display myChemistry directly on the web site using QuickLaTeX. For some reason though, it’s not possible to use it directly in the exact meaning of the word.
This code
$
[preamble]
\usepackage{mychemistry}
[/preamble]
\begin{rxn}
\reactant{\chemfig{*6(=-=-=-)}} \arrow[,,1.5]{\ce{Br2}, \ce{AlBr3}}{\ce{- HBr}}\reactant{\chemfig{*6(=-=(-Br)-=-)}}
\end{rxn}
$
[preamble]
\usepackage{mychemistry}
[/preamble]
\begin{rxn}
\reactant{\chemfig{*6(=-=-=-)}} \arrow[,,1.5]{\ce{Br2}, \ce{AlBr3}}{\ce{- HBr}}\reactant{\chemfig{*6(=-=(-Br)-=-)}}
\end{rxn}
$
will fail:

If you put it inside of a node of a tikzpicture, however, it is working:
[preamble]
\usepackage{mychemistry}
[/preamble]
\begin{tikzpicture}
\node{
\begin{rxn}
\reactant{\chemfig{*6(=-=-=-)}} \arrow[,,1.5]{\ce{Br2}, \ce{AlBr3}}{\ce{- HBr}}\reactant{\chemfig{*6(=-=(-Br)-=-)}}
\end{rxn}
};
\end{tikzpicture}
\usepackage{mychemistry}
[/preamble]
\begin{tikzpicture}
\node{
\begin{rxn}
\reactant{\chemfig{*6(=-=-=-)}} \arrow[,,1.5]{\ce{Br2}, \ce{AlBr3}}{\ce{- HBr}}\reactant{\chemfig{*6(=-=(-Br)-=-)}}
\end{rxn}
};
\end{tikzpicture}

Update 2011/05/09: Pavel Holoborodko, the developer of QuickLaTeX told me (a) that he is going to add support for direct use of ChemFig and myChemistry in the next release of QuickLaTeX and (b) a slightly simpler workaround:
[latex]
%\begin{tikzpicture}\end{tikzpicture}
\begin{rxn}
\mesomeric{
\reactant{
\chemfig{*6(=[@{e1}]-=-(-[:120]X)(-[:60]H)-(-[:-30,.4,,,white]\oplus)-[@{e2}])}
\elmove{e1}{60:4mm}{e2}{0:4mm}
}
\marrow
\reactant{
\chemfig{*6(-(-[:90,.4,,,white]\oplus)-[@{e4}]=[@{e3}]-(-[:120]X)(-[:60]H)-=)}
\elmove{e3}{180:4mm}{e4}{150:4mm}
}
\marrow
\reactant{
\chemfig{*6(-=-(-[:-150,.4,,,white]\oplus)-(-[:120]X)(-[:60]H)-=)}
}
}
\end{rxn}
[/latex]
%\begin{tikzpicture}\end{tikzpicture}
\begin{rxn}
\mesomeric{
\reactant{
\chemfig{*6(=[@{e1}]-=-(-[:120]X)(-[:60]H)-(-[:-30,.4,,,white]\oplus)-[@{e2}])}
\elmove{e1}{60:4mm}{e2}{0:4mm}
}
\marrow
\reactant{
\chemfig{*6(-(-[:90,.4,,,white]\oplus)-[@{e4}]=[@{e3}]-(-[:120]X)(-[:60]H)-=)}
\elmove{e3}{180:4mm}{e4}{150:4mm}
}
\marrow
\reactant{
\chemfig{*6(-=-(-[:-150,.4,,,white]\oplus)-(-[:120]X)(-[:60]H)-=)}
}
}
\end{rxn}
[/latex]

You can use this in comments as well. :-)
May 11, 2011 at 12:30
Pavel Holoborodko really is a fast guy. Just a day later he released an update to QuickLaTeX, that naturally supports ChemFig and myChemistry. We now don’t need the tikzpicture workaround any more!