People have mentioned flex and grid a few times. I would add using box-sizing: border-box; solves a lot of problems stemming css' default of measuring from the

People have mentioned flex and grid a few times. I would add using box-sizing: border-box; solves a lot of problems stemming css' default of measuring from the inside of elements that need to fit together from their outsides. See https://css-tricks.com/almanac/properties/b/box-sizing/ https://css-tricks.com/almanac/properties/b/box-sizing/