Why are margin/padding percentages in CSS always calculated against width? (stackoverflow.com)

I discovered this CSS oddity while trying to figure out why my plan to use (negative) margins to create a new "effective" content box wasn't working with percent units (but worked with other units).

The best explanation seems to be in the middle of this article: https://www.aleksandrhovhannisyan.com/blog/css-aspect-ratio/...