\begin{tikzpicture}
	\tikzset{
		mstyle/.style={column sep=-\pgflinewidth,row sep=-\pgflinewidth,font=\footnotesize,},
		window/.style={draw,very thick,blue},
	}
	\matrix(convmat)[matrix of nodes,ampersand replacement=\&,column sep=4\pgflinewidth,row sep=3\pgflinewidth,
	nodes={draw,rectangle, minimum width=8mm, minimum height=8mm,font=\footnotesize,anchor=south}]{
		a \& b \& c \& d \\
		e \& f \& g \& h \\
		i \& j \& k \& l \\
	};
	\foreach \j/\k [count=\i] in {2/8,3/9,4/10}{
		\onslide<\k>{
			\draw[window](convmat-1-\i.north west)rectangle(convmat-2-\j.south east);
	}}
	\foreach \j/\k [count=\i] in {2/11,3/12,4/13}{
		\onslide<\k>{
			\draw[window](convmat-2-\i.north west)rectangle(convmat-3-\j.south east);
	}}
\end{tikzpicture}