What the site fails to explain is why these things are important. Changing a property that effects a layout change means the browser will need to redraw that element and anything that the geometry has pushed around. A full repaint is slow so if you're doing it often the user's experience will be very negatively impacted. A composite event should be pretty fast as it happens largely on the GPU hardware.

For this reason, for example, animating things with a CSS transform is considerably better for the user than animating a property like left or margin-left. Animating with CSS can be much better than something like jQuery's animate().

If you want to know more, google 'layout thrashing'.