What would be the best way to achieve this result? Using CSS and / or JS.
Each successive row of elements, buttons, in this case, is smaller than the previous.
most likely CSS, perhaps with some JS? - do you have a link to where you saw this? You can examine the properties with the devtools and see what styles are being applied.
I wish. 
There’s only this image. This does feel like an exercise from a computer science book for extra credit.