My observation is that stacking gel is still about 4% acrylamide (typically), and you could separate proteins with the right equipment using only the 4% in theory. The reason things “stack” is that the relative rate of the small proteins in a 10-20% acrylamide matrix is less than (ish?) that of the largest protein rate moving through the 4% portion. Once the largest catch up, they move slower also, and separation begins. I suppose the quality of stacking depends on how different you can make your matrices, although I’ve found it hard to make less than a 4% gel that holds its form.

In the video, their pictorial of proteins migrating in the stacking gel shows the smallest proteins stationary in the middle of the stacking phase, and only moving as the larger ones reach them, when in fact they all move as described above.