CSS2.1 says that there are two types of length units: relative and absolute (both represented by a "length" data type). Relative length units specify a length relative to another length property. Style sheets that use relative units will more easily scale from one medium to another. Relative units are: em, ex, px. Absolute length units are only useful when the physical properties of the output medium are known. Absolute length units are: in, cm, mm, pt, pc. (ref. Section 4.3.3 of CSS2.1 spec). CSS 2.1 properties which support length units (of both types) are: background-position, background, border-spacing, border-top-width, border-bottom-width, border-left-width, border-right-width, border-width, border-top, border-bottom, border-right, border-left, border, bottom, font-size, font, height, left, letter-spacing, line-height, margin-width, margin-left, margin-top, margin-bottom, margin-right, margin, max-height, max-width, min-height, min-width, outline-width, outline, padding-top, padding-right, padding-bottom, padding-left, padding, right, text-indent, top, vertical-align, width, word-spacing. With any of these one can use either relative or absolute length units (differentiated in CSS grammar by CSS parser).