The width you declare doesn't always equal the width you see. With content-box sizing, padding and borders are added outside the declared content width. Border-box sizing includes them, which is often easier for cards that must fit a known space. Margins live outside the border and separate siblings; padding creates room inside it. You'll compare border boxes in the preview instead of judging them by eye. Long unbroken content adds a different problem: it can overflow a perfectly sized container. Keep the text visible and let it wrap. Hiding content merely conceals the sizing mistake.