Can I redefine \begin so that it does not complain about undefined environment

Question

I'd like to be able to do the following kind of thing:

\documentclass{standalone}
\begin{document}
Foo
\begin{MyUndefinedEnv}
  Bar
\end{MyUndefinedEnv}
\end{document}

without LaTeX complaining about undefined environment. Is it possible without breaking existing environments?

Answer 1

Yes, you can:

\documentclass{article}

\makeatletter
\let\xbegin\begin % store original \begin
\let\xifundefined\@ifundefined % store original \@ifundefined
\def\begin{%
  % "inactivate" \@ifundefined, but only once, hence reverting
  % it to original definition immediately
  \def\@ifundefined##1##2##3{\global\let\@ifundefined\xifundefined##3}%
  \xbegin}
\makeatother

\begin{document}

\begin{ttgg}
Hello!
\end{ttgg}

\begin{center}
Hello!
\end{center}

\end{document}