A BorderMatrix

This is a 2x2 mtable in which the cells — except for cell (1,1), which is empty — are themselves mtables. CSS is used to control column alignment between the two inner mtables on the right by setting the width property for the cells of those inner mtables.

$$\begin{array}{cc}\hfill \hfill & \hfill \begin{array}{ccc}\hfill \text{a}\hfill & \hfill \text{b}\hfill & \hfill \text{c}\hfill \end{array}\hfill \\ \hfill \begin{array}{c}\hfill \text{x}\hfill \\ \hfill \text{y}\hfill \\ \hfill \text{z}\hfill \end{array}\hfill & \hfill \left(\begin{array}{ccc}\hfill 1\hfill & \hfill 2\hfill & \hfill 3\hfill \\ \hfill \hfill & \hfill 22\hfill & \hfill 33\hfill \\ \hfill \hfill & \hfill \hfill & \hfill 333\hfill \end{array}\right)\hfill \end{array}$$