31.1 Parts

<..book / report / article..>+
 \ifx \part\:UnDef\else
    <.html late parts.>
 \fi
 -_-_-

<..html late parts..>
 \def\@part[#1]#2{%
     \ifnum \c@secnumdepth >-2\relax
       \SkipRefstepAnchor \refstepcounter{part}%
       \addcontentsline{toc}{part}{\thepart\hspace{1em}#1}%
     \else
       \addcontentsline{toc}{part}{#1}%
     \fi
     \csname @endpart\endcsname}
 -_-_-

The \@endpart is not defined for ‘article’ class.

The above is LaTeX’s def without the output of titles.

<..html late parts..>+
 \let\:tempb=\part
 \Def:Section\part{\thepart}{#1}
 \let\:part=\part
 \let\part=\:tempb
 \let\no@part=\@part
 \def\@part[#1]#2{%
    {\let\addcontentsline=\:gobbleIII\no@part[#1]{}}%
    \HtmlEnv   \Toc:Title{#1}\:part{#2}}
 -_-_-

<..html late parts..>+
 \Def:Section\likepart{}{#1}
 \let\:likepart=\likepart
 \let\likepart=\:UnDef
 \let\no@spart=\@spart
 \def\@spart#1{%
    {\let\addcontentsline=\:gobbleIII\no@spart{}}%
    \HtmlEnv   \:likepart{#1}}
 -_-_-