Line breaks \\ in , for instance, the ‘verse’ environment are requested indirectly through the following.
<..html latex env..>+
\def\@centercr{\ifhmode \unskip\else \@nolnerr\fi
\ifx \EndPicture\:UnDef \a:centercr \b:centercr
\fi \par \@ifstar{\nobreak\@xcentercr}\@xcentercr}
-_-_-
<..html latex env..>+
\def\@icentercr[#1]{%
\ifx \EndPicture\:UnDef
\ifdim #1>0.5\baselineskip \a:centercr\fi
\fi \vskip #1\ignorespaces}
-_-_-
<..config latex.ltx utilities..>+
\NewConfigure{centercr}{2}
-_-_-
Where ‘<P>’ comes from???