Hi Nasser,
> ==================================== > (/usr/local/texlive/2019/texmf-dist/tex/generic/tex4ht/html5.4ht)) (./foo.aux) > ! Undefined control sequence. > \\ ->\let \reserved@e > \relax \let \reserved@f \relax \@ifstar {\let > \reserv... > > l.101 \EndPreamble > ======================= I don't get any error when I compile your mwe, it just doesn't work. It works when I use the URL you pass in `\Configure{@HEAD}` to \Configure{MathjaxSource}: \Configure{MathjaxSource}{https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js} I am bit surprised that I don't need to use \detokenize around TeX macros in Mathjax configuration. I used some of your declared macros and they seem to work. Best regards, Michal