Because browsers themselves have set that expectation by accepting unitless 0 in most (all?) other places in CSS. e.g. "margin: 3px 0 4em 0" is a valid way to

Because browsers themselves have set that expectation by accepting unitless 0 in most (all?) other places in CSS. e.g. "margin: 3px 0 4em 0" is a valid way to set the right and left margins to 0px.