Several attributes specify widths as a number followed by an optional suffix. The units for widths are specified by the suffix: pt denotes points, pi denotes picas, in denotes inches, cm denotes centimeters, mm denotes millimeters, em denotes em units (equal to the height of the default font), and px denotes screen pixels. The default units are screen pixels (chosen for backwards compatibility). The number is an integer value or a real valued number such as "2.5". Exponents, as in "1.2e2", are not allowed. White space is not allowed between the number and the suffix.
The above set of suffices is augmented for certain elements: "%" is used for the WIDTH attribute for the TABLE element. It indicates that the attribute specifies the percentage width of the space between the current left and right margins, e.g. width="50%". For the COL element, "*" is used with the WIDTH attribute to specify relative column widths, e.g. width="3*", using the same representation as the CALS table model.