LaTeX disables: ‘\tabs’, ‘\tabsdone’, settabs, \+, \tabset, \cleartabs, and \settabs.
No need for ‘\:CheckOption{settabs-} \if:Option \else ....\fi’ because we can use ‘\Configure{settabs}{}{}{}{}{}’.
The following is from plain TeX.
The following is for ‘\outer\def\+{\settabs}’.
<..html tex settabs..>
\def\gt:tab{\def\:temp##1.##2//{##1}\tmp:dim=\m:tab\wd\z@}
<.set a settabs element.>
<.set a settabs row.>
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<..config plain utilities..>+
\def\E:tabalign#1{\def\e:tabalgn{\ifx \EndPicture\:UnDef #1\fi}}
\def\c:settabs:{\c:def\:tempa{settabs}\futurelet\:temp\:tblgn}
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<..html tex settabs..>+
\pend:def\settabs{\e:tabalgn}
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The \leavevmode below save from paragraph breaks between lines.
‘\Configure{settabs}[dec const]’ for table magnified by the given decimal constant. ‘\Configure{settabs}{at-settabs}{at \+}{at \cr}{before elem}{after elem}’ for user-defined configuration. The ‘at-settabs’ is useful for tables ‘\settabs..\+..\cr\+..\cr...’ that are defined within dedicated groups.
<..set a settabs element..>
\def\t@bb@x{\if@cr\egroup <.last col; no dim.>%
\else\hss\egroup <.col dim.>%
\global\setbox\tabsyet\hbox{\unhbox\tabsyet
\global\setbox\@ne\lastbox}%
\ifvoid\@ne\global\setbox\@ne\hbox to\wd\z@{}%
\else\setbox\z@\hbox to\wd\@ne{\unhbox\z@}\fi
\global\setbox\tabsdone\hbox{\box\@ne\unhbox\tabsdone}\fi
\ifx \EndPicture\:UnDef \c:tabalgn \fi \box\z@
\ifx \EndPicture\:UnDef \d:tabalgn \fi}
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<..col dim..>
\edef\TabWidth{\expandafter\:temp\the\tmp:dim//}%
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<..last col; no dim..>
\let\TabWidth=\empty
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<..set a settabs row..>
\def\m@ketabbox{\begingroup
\global\setbox\tabsyet\copy\tabs
\global\setbox\tabsdone\null
\def\cr{\@crtrue\crcr\egroup\egroup
\ifus@ \ifx \EndPicture\:UnDef \a:tabalgn \fi
\unvbox\z@\lastbox
\ifx \EndPicture\:UnDef \b:tabalgn \fi
\fi\endgroup
\setbox\tabs\hbox{\unhbox\tabsyet\unhbox\tabsdone}}%
\setbox\z@\vbox\bgroup\@crfalse
<.ialign.>\bgroup&\t@bbox##\t@bb@x\crcr}
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\ialign stands for initialized \halign.