box-sizing: border-box;

box-sizing: content-box;

You do not need to declare box-sizing:content-box as it is implied as soon as you specify a width or height using CSS. Declaring box-sizing:content-box means that matching elements will have "layout" (this is exactly what the "Holly Hack" does). You can mix and match measurement units (cm, em, px etc) as this example demonstrates.

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