The original for the following is ‘\def\centerline#1{\line{\hss#1\hss}}’. We replace the \line with \hbox, because the earlier produces html code of itself, and surrounding horizontal space from its definition as \hbox to \hsize. Similarly is the case for \leftline and \rightline.
<..html tex env..>
\pend:defI\centerline{\pic:gobble\cnt:a}
\append:defI\centerline{\pic:gobble\cnt:b}
\pend:defI\leftline{\pic:gobble\lft:a}
\append:defI\leftline{\pic:gobble\lft:b}
\pend:defI\rightline{\pic:gobble\a:rightline}
\append:defI\rightline{\pic:gobble\b:rightline}
-_-_-
In the ‘sty’ mode we have the following configuration. Otherwise, the configuration is empty.
<..config plain utilities..>+
\NewConfigure{centerline}[2]{\c:def
\cnt:a{\a:leftline{#1}}\c:def\cnt:b{\b:leftline{#2}}}
\NewConfigure{leftline}[2]{\c:def
\lft:a{\a:leftline{#1}}\c:def\lft:b{\b:leftline{#2}}}
\NewConfigure{rightline}[2]{\c:def
\a:rightline{\a:leftline{#1}}\c:def\b:rightline{\b:leftline{#2}}}
\long\def\a:leftline#1{{\ht:everypar{}#1}\expandafter\hbox\:gobble}
\long\def\b:leftline#1{{\ht:everypar{}#1}}
-_-_-