CSS Units - In CSS, length is measured by either an absolute value or a relative value. An absolute unit is a fixed value such as pixel, point, inches, etc while a relative unit such as %, em etch is relative to the parent element.
Please keep in mind that, the CSS unit is applicable only for a non-zero value. Specify length with value including its unit otherwise, it would not work. If the length value is zero(0) then, in this case, you can remove the CSS unit.