The computed value of a CSS property is computed from the specified value by:
- handling the special values
- doing the computation needed to reach the value described in the "Computed value" line in the property's summary.
The computation needed to reach the "Computed value" for the property typically involves converting relative values (such as those in 'em' units or percentages) to absolute values.
For example, if an element has specified values
font-size: 16px and
padding-top: 2em, then the computed value of
32px (double the font size).
However, for some properties (those where percentages are relative to something that may require layout to determine, such as width, margin-right, text-indent, and top), percentage specified values turn into percentage computed values. Additionally, unitless numbers specified on the line-height property become the computed value, as specified. These relative values that remain in the computed value become absolute when the used value is determined.