How to highlight math environments with colored background?

Question

How to highlight maths environments (in-line mode excluded) with a coloured rectangular background e.g. grey?

A \textwidth rectangular behind the \begin{equation} environment.

Answer 1

It is possible to use mdframed and surround the equation environments.

\documentclass{article}

\usepackage{mdframed}
\usepackage{xcolor}
\surroundwithmdframed[
    hidealllines=true,
    backgroundcolor=black!20,
    skipbelow=\baselineskip,
    skipabove=\baselineskip
]{equation}

\begin{document}
Text
\begin{equation}
1+1=2
\end{equation}
Text
\end{document}

Answer 2

I think I really didn't follow

The rectangular is expected to be below the equation and also the equation counter. In better words \textwidth.

Hence this answer ;) I don't know to what extent this will be useful to anybody. But for the sake of it, I present the following:

This solution uses etoolbox with its \AfterEndEnvironment macro.

\documentclass{article}
\usepackage{tikz,lipsum}
\usepackage{etoolbox}
\AfterEndEnvironment{equation}{{\tikz\draw[fill=black!20,draw=none] (0,0) rectangle (\textwidth, .3);}\par}


\begin{document}
\lipsum[1]
\begin{equation}
1+1=2
\end{equation}
\lipsum[2]
\begin{equation}
1+1=2
\end{equation}
\lipsum[3]
\end{document}