It seems that configuration for the Natbib package doesn't take into account the \Configure{cite} command. I've just added the support for that. You can use the following patched natbib.4ht file until the support is added to TL:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% natbib.4ht 2019-07-26-22:42 %
% Copyright (C) 1999-2009 Eitan M. Gurari %
% Copyright 2009-2019 TeX Users Group
%
% This work may be distributed and/or modified under the
% conditions of the LaTeX Project Public License, either
% version 1.3c of this license or (at your option) any
% later version. The latest version of this license is in
% http://www.latex-project.org/lppl.txt
% and version 1.3c or later is part of all distributions
% of LaTeX version 2005/12/01 or later.
%
% This work has the LPPL maintenance status "maintained".
%
% The Current Maintainer of this work
% is the TeX4ht Project <http://tug.org/tex4ht>.
%
% If you modify this program, changing the
% version identification would be appreciated.
\HRestore\@lbibitem
\HRestore\@bibitem
\ifNAT@super
\def\citet{\@ifstar{\NAT@longnamestrue\ht@citet}%
{\NAT@longnamesfalse\ht@citet}}
\def\setb:anc#1{\let\sv:anc\AnchorLabel
\def\bib:anc{\Link{}{bibitem\the
\c@NAT@ctr}\EndLink \gdef\bib:anc{}}%
\def\AnchorLabel{\bib:anc}}
\def\nb:link#1{\Protect\Link{bibitem#1}{}#1\Protect\EndLink}
\def\nb@link#1{\Protect\Link{bibitem#1}{}#1\Protect\EndLink}
\let\nat:lbibitem\@lbibitem
\def\@lbibitem{%
\ifx\hyper@natanchorstart\@gobble
\def\hyper@natanchorstart##1{\Link{}{bibitem\the
\c@NAT@ctr}\EndLink}\fi
\nat:lbibitem
}
\catcode`\:12
\def\ht@citet#1{\def\NAT@num{-1}\let\NAT@last@yr\relax\let\NAT@nm\@empty
\let\@citea\@empty
\@for\@citeb:=#1\do{%
\@ifundefined{b@\@citeb\@extra@b@citeb}%
{{\reset@font\bfseries?}\NAT@citeundefined\PackageWarning{natbib}%
{Citation `\@citeb' on page \thepage \space undefined}%
}%
{\@citea\let\NAT@last@num\NAT@num\let\NAT@last@nm\NAT@nm
\NAT@parse{\@citeb}%
\ifNAT@longnames\@ifundefined{bv@\@citeb\@extra@b@citeb}{%
\let\NAT@name=\NAT@all@names
\global\@namedef{bv@\@citeb\@extra@b@citeb}{}}{}%
\fi
\ifNAT@full\let\NAT@nm\NAT@all@names\else
\let\NAT@nm\NAT@name
\fi
\NAT@nm}\Tg<sup>\nb@link{\NAT@num}\Tg</sup>
\def\@citea{\unskip\NAT@sep\penalty\@m\space}%
}%
}
\def\NAT@citexnum[#1][#2]#3{%
\NAT@sort@cites{#3}%
\let\@citea\@empty
\@cite{\def\NAT@num{-1}\let\NAT@last@yr\relax\let\NAT@nm\@empty
\@for\@citeb:=\NAT@cite@list\do
{\edef\@citeb{\expandafter\@firstofone\@citeb}%
\if@filesw\immediate\write\@auxout{\string\citation{\@citeb}}\fi
\@ifundefined{b@\@citeb\@extra@b@citeb}{%
{\reset@font\bfseries?}
\NAT@citeundefined\PackageWarning{natbib}%
{Citation `\@citeb' on page \thepage \space undefined}}%
{\let\NAT@last@num\NAT@num\let\NAT@last@nm\NAT@nm
\NAT@parse{\@citeb}%
\ifNAT@longnames\@ifundefined{bv@\@citeb\@extra@b@citeb}{%
\let\NAT@name=\NAT@all@names
\global\@namedef{bv@\@citeb\@extra@b@citeb}{}}{}%
\fi
\ifNAT@full\let\NAT@nm\NAT@all@names\else
\let\NAT@nm\NAT@name\fi
\ifNAT@swa
\ifnum\NAT@ctype=2\relax\@citea
\hyper@natlinkstart{\@citeb\@extra@b@citeb}%
\NAT@test{2}\hyper@natlinkend\else
\ifnum\NAT@sort>1
\begingroup\catcode`\_8
\ifcat _\ifnum\z@<0\NAT@num _\else A\fi
\global\let\NAT@nm=\NAT@num \else \gdef\NAT@nm{-2}\fi
\ifcat _\ifnum\z@<0\NAT@last@num _\else A\fi
\global\@tempcnta\NAT@last@num
\global\advance\@tempcnta \@ne
\else \global\@tempcnta\m@ne\fi
\endgroup
\ifnum\NAT@nm=\@tempcnta
\ifx\NAT@last@yr\relax
\edef\NAT@last@yr{\@citea
\csname o:mbox:\endcsname{\csname nb:link\endcsname
\NAT@num}}%
\else
\edef\NAT@last@yr{--\penalty
\@m\csname o:mbox:\endcsname{\csname nb:link\endcsname
\NAT@num}}%
\fi
\else
\NAT@last@yr \@citea \csname o:mbox:\endcsname{\csname nb:link\endcsname
\NAT@num}%
\let\NAT@last@yr\relax
\fi
\else
\@citea \csname o:mbox:\endcsname
{\ifx\hyper@natanchorstart\@gobble
\Link{bibitem\NAT@num}{}\NAT@num
\EndLink
\else
\hyper@natlinkstart{\@citeb\@extra@b@citeb}\NAT@num
\hyper@natlinkend
\fi
}%
\fi
\fi
\def\@citea{\NAT@sep\penalty\@m\NAT@space}%
\else
\ifcase\NAT@ctype\relax
\ifx\NAT@last@nm\NAT@nm \NAT@yrsep\penalty\@m\NAT@space\else
\@citea \NAT@test{1}\ \NAT@@open
\if\relax#1\relax\else#1\ \fi\fi \NAT@mbox{%
\hyper@natlinkstart{\@citeb\@extra@b@citeb}%
\NAT@num\hyper@natlinkend}%
\def\@citea{\NAT@@close\NAT@sep\penalty\@m\ }%
\or\@citea
\hyper@natlinkstart{\@citeb\@extra@b@citeb}%
\NAT@test{1}\hyper@natlinkend
\def\@citea{\NAT@sep\penalty\@m\ }%
\or\@citea
\hyper@natlinkstart{\@citeb\@extra@b@citeb}\NAT@test{2}%
\hyper@natlinkend
\def\@citea{\NAT@sep\penalty\@m\ }%
\fi
\fi
}}%
\ifnum\NAT@sort>1\NAT@last@yr\fi
\ifNAT@swa\else\ifnum\NAT@ctype=0\if\relax#2\relax\else
\NAT@cmt\ #2\fi \NAT@@close\fi\fi}{#1}{#2}}
\let\@citex\NAT@citexnum
\catcode`\:11
\else
\def\hyper@natlinkstart#1{%
\let\rel:hyper\def \hyper@linkstart{cite}{X#1}%
\def\hyper@nat@current{#1}%
}
\def\hyper@natlinkbreak#1#2{%
\hyper@linkend#1\let\rel:hyper\def \hyper@linkstart{cite}{X#2}%
}
\def\hyper@natlinkend{\hyper@linkend}
\expandafter\ifx \csname hyper@linkstart\endcsname\relax
\def\hyper@linkstart#1#2{%
\a:cite%
\def\:temp{#1}%
\ifx\:temp\@urltype
\cIteLink[#2]{}{}%
\else
\ifx\rel:hyper\def\cIteLink{#2}{}\else\cIteLink[\##2]{}{}\fi
\fi \global\let\rel:hyper=\:UnDef
}
\def\hyper@linkend{\EndcIteLink\b:cite}
\fi
\long\def\:temp#1{}\ifx \:temp\hyper@natanchorstart
\def\hyper@natanchorstart#1{\Link{}{X#1}\EndLink}
\fi
\pend:defIII\NAT@cite{\let\mbox\o:mbox:}
\pend:defIII\NAT@citenum{\let\mbox\o:mbox:}
\fi
\Hinput{natbib}
\endinput
The important change is this:
\def\hyper@linkstart#1#2{%
\a:cite%
\def\:temp{#1}%
\ifx\:temp\@urltype
\cIteLink[#2]{}{}%
\else
\ifx\rel:hyper\def\cIteLink{#2}{}\else\cIteLink[\##2]{}{}\fi
\fi \global\let\rel:hyper=\:UnDef
}
\def\hyper@linkend{\EndcIteLink\b:cite}
The \a:cite and \b:cite commands add the tags around the individual citations, \cIteLink is a custom link command, which needs to be configured correctly:
\LinkCommand\Xreflink{xref,id,rid,ref-type="bibr"}
\Configure{cite}{}{}
{\Xreflink}{\EndXreflink}
The link command used in fourth argument of \Configure{cite} must be defined using the \LinkCommand command. It takes arguments separated by commas:
\LinkCommand...................1 <= i <= 6
Creates a \Link-like command
#1 tag name
#2 href-like attribute
#3 name-like attribute
#4 insertion
#5 /, if empty element
#6 replacement for # (ignored if absent)
Example:
\LinkCommand\JSLink{a,\noexpand\jsref,name}
\def\jsref="#1"{href="javascript:window.open('#1')"}
\JSLink{a}{}xx\EndJSLink
\Link{}{a}\EndLink % or \JSLink{}{a}\EndJSLink
This is the result:
<p class="indent" > Sophisticated macro scripts involving loops and conditional statements are,
admittedly, hard to write and read, and it may take some time to become familiar with it
[<xref
id="Xbib1" ref-type="bibr" >1</xref>].
</p>
