\tl_const:Nx \c_stex_html_emptyarg_tl {~}
\cs_new_protected:Nn \stex_annotate:nnn {#3}
\cs_new_protected:Nn \stex_annotate_invisible:n {}
\cs_new_protected:Nn \stex_annotate_invisible:nnn {}
\NewDocumentEnvironment{stex_annotate_env} { m m } {}{}
\prg_new_conditional:Nnn \stex_html_backend: {p, T, F, TF} {
\prg_return_false:
}