That's where you start. But to make it responsive it takes more work. If I remember..
For a 9x9 basic table grid you would set the width 100% of the top and bottom image do the opposite for the sides and fix the corners. All of these tricks involved choosing designs that could work.
It never worked well. The html was full of issues and the kind of image slicing was for backwards compatibility with email clients. However PS did have a way to write scripts with JS and extend or even change the HTML generation.
For a 9x9 basic table grid you would set the width 100% of the top and bottom image do the opposite for the sides and fix the corners. All of these tricks involved choosing designs that could work.
It would have been a tool I would have liked.