Are you trying to make a framed section-like title?
Because then, make a float (a table
) is the worse idea possible. This can displace the title to anywhere. Although with [H]
option you make that the float does not float, still a table
environment is completely unnecessary. May be you are using it to avoid indentation of the table a add vertical spaces, but this can be solved better without a float (respectively, with \noindent
and some vertical spacing command as \bigskip
, or maybe set \parindent
to 0pt and \parskip
to some more).
Nested tabulars do not make sense either. One (simple) tabular is enough, or a framed box (\fbox{}
) that can take the line width less the space used by the framed box, as a \makebox
, a \parbox
, or a minipage environement, may be in a section redefinition or alternatively, use a very configurable boxing package as mdframed
or tcolorbox
, or simply write some \vrules and \hrules around, without making any box.
The example show 5 of the many alternatives. Use the one that makes you happy.

\documentclass{article}
% Padding of \fbox{}
\setlength\fboxsep{.5em}
% Some dummy text between solutions, just to see the layout
\def\loreipsum{Lorem ipsum dolor sit amet, consectetur adipiscing
elit. Fusce elit lectus, dapibus eget pellentesque eu, ultricies
vel turpis. Cras quis convallis nibh.}
% for solution 2 only
\makeatletter
\newcommand\fsection{\@startsection{section}{1}{\z@}%
{1em}{1em}{\bfseries\framedbox}}
\makeatother
\newcommand*\framedbox[1]{\noindent%
\fbox{\parbox{\dimexpr\columnwidth-2\fboxsep}{#1}}}
% for solution 5 only
\usepackage{mdframed}
\begin{document}
\loreipsum
% solution 1
\bigskip\hrule
\noindent
\vrule height 12pt depth 6pt width 0.4pt
{ \bfseries Description sommaire}\hfill\vrule
\hrule\bigskip
\loreipsum
% solution 2
\fsection*{Description sommaire}
\loreipsum
% solution 3
\bigskip
\noindent\fbox{\makebox[\dimexpr\linewidth-2\fboxsep-2\fboxrule][l]
{\textbf{Description sommaire}}}\bigskip
\loreipsum
% solution 4
\bigskip
{\renewcommand\arraystretch{1.4}\setlength\tabcolsep{.5em}
\noindent\begin{tabular}%
{|p{\dimexpr\linewidth-2\tabcolsep-2\arrayrulewidth}|}\hline
\textbf{Description sommaire} \\\hline
\end{tabular}}
\bigskip
\loreipsum
% solution 5
\begin{mdframed}
\textbf{Description sommaire}:
\end{mdframed}
\loreipsum
\end{document}
tabular*
environment to achieve your typesetting objective.tabular*
is designed to achieve the desired width (here:\textwidth
) by dynamically enlarging the amount of intercolumn whitespace. For this setup to have any chance to work, there must be at least two columns. You really should be looking into using atabularx
environment.