\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: }