Turn on box-sizing border-box before your first layout

You need

A layout with at least one box, such as a card or an input, that needs to keep an exact width even after you add padding or a border, plus a stylesheet you can edit before your other rules run.

Not covered

This does not cover the gap margin collapse leaves between stacked elements, or choosing Flexbox over Grid for the layout itself; it only covers whether a declared width survives padding and border.